Display device, display module, and electronic device

ABSTRACT

A display device includes a liquid crystal element, a transistor, a scan line, and a signal line. The liquid crystal element includes a pixel electrode, a liquid crystal layer, and a common electrode. The scan line and the signal line are each electrically connected to the transistor. The scan line and the signal line each include a metal layer. The transistor is electrically connected to the pixel electrode. A semiconductor layer of the transistor includes a stack of a first metal oxide layer and a second metal oxide layer. The first metal oxide layer includes a region with lower crystallinity than the second metal oxide layer. The transistor includes a first region connected to the pixel electrode. The pixel electrode, the common electrode, and the first region are each configured to transmit visible light. Visible light passes through the first region and the liquid crystal element and exits from the display device.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a liquid crystaldisplay device, a display module, and an electronic device.

Note that one embodiment of the present invention is not limited to theabove technical field. Examples of the technical field of one embodimentof the present invention include a semiconductor device, a displaydevice, a light-emitting device, a power storage device, a memorydevice, an electronic device, a lighting device, an input device (suchas a touch sensor), an input/output device (such as a touch panel), amethod for driving any of them, and a method for manufacturing any ofthem.

2. Description of the Related Art

Transistors used for most flat panel displays typified by a liquidcrystal display device and a light-emitting display device are formedusing silicon semiconductors such as amorphous silicon, single crystalsilicon, and polycrystalline silicon provided over glass substrates.Further, such a transistor employing such a silicon semiconductor isused in integrated circuits (ICs) and the like.

In recent years, attention has been drawn to a technique in which,instead of a silicon semiconductor, a metal oxide exhibitingsemiconductor characteristics is used in transistors. Note that in thisspecification, a metal oxide exhibiting semiconductor characteristics isreferred to as an oxide semiconductor. For example, in Patent Documents1 and 2, a technique is disclosed in which a transistor is manufacturedusing zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductorand the transistor is used as a switching element or the like of a pixelof a display device.

REFERENCE Patent Documents

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

One object of one embodiment of the present invention is to provide aliquid crystal display device with a high aperture ratio. One object ofone embodiment of the present invention is to provide a liquid crystaldisplay device with low power consumption. One object of one embodimentof the present invention is to provide a high-definition liquid crystaldisplay device. One object of one embodiment of the present invention isto provide a highly reliable liquid crystal display device.

Note that the description of these objects does not disturb theexistence of other objects. One embodiment of the present invention doesnot necessarily achieve all the objects. Other objects can be derivedfrom the description of the specification, the drawings, and the claims.

One embodiment of the present invention is a display device including aliquid crystal element, a transistor, a scan line, and a signal line.The liquid crystal element includes a pixel electrode, a liquid crystallayer, and a common electrode. The scan line and the signal line areeach electrically connected to the transistor. The scan line and thesignal line each include a metal layer. The transistor is electricallyconnected to the pixel electrode. A semiconductor layer of thetransistor includes a stack of a first metal oxide layer and a secondmetal oxide layer. The first metal oxide layer includes a region withlower crystallinity than the second metal oxide layer. The transistorincludes a first region connected to the pixel electrode. The pixelelectrode, the common electrode, and the first region are eachconfigured to transmit visible light. Visible light passes through thefirst region and the liquid crystal element and exits from the displaydevice.

One embodiment of the present invention is a display device including aliquid crystal element, a transistor, a scan line, and a signal line.The liquid crystal element includes a pixel electrode, a liquid crystallayer, and a common electrode. The scan line and the signal line areeach electrically connected to the transistor. The scan line and thesignal line each include a metal layer. The transistor is electricallyconnected to the pixel electrode. The transistor includes a gateelectrode, an insulating layer over the gate electrode, a semiconductorlayer over the insulating layer, and a pair of electrodes over thesemiconductor layer. The semiconductor layer includes a first metaloxide layer and a second metal oxide layer over the first metal oxidelayer. The first metal oxide layer includes a region with lowercrystallinity than the second metal oxide layer. The transistor includesa first region connected to the pixel electrode. The pixel electrode,the common electrode, and the first region are each configured totransmit visible light. Visible light passes through the first regionand the liquid crystal element and exits from the display device.

It is preferable that the first metal oxide layer and the second metaloxide layer each independently include indium, metal M (M representsaluminum, gallium, yttrium, or tin), and zinc. For example, an atomicratio of the indium to the metal M and the zinc is 4:x:y, where x isgreater than or equal to 1.5 and less than or equal to 2.5 and y isgreater than or equal to 2 and less than or equal to 4. For example, anatomic ratio of the indium to the metal M and the zinc is 5:x:y, where xis greater than or equal to 0.5 and less than or equal to 1.5 and y isgreater than or equal to 5 and less than or equal to 7.

It is preferable that the second metal oxide layer include a crystalpart having c-axis alignment.

The display device with the above-described structure may furtherinclude a touch sensor. The touch sensor is closer to a display surfacethan the liquid crystal element and the transistor are.

It is preferable that the scan line include a portion overlapping withthe semiconductor layer.

Visible light may pass through the first region and the liquid crystalelement in the order presented and exit from the display device.Alternatively, visible light may pass through the liquid crystal elementand the first region in the order presented and exit from the displaydevice.

It is preferable that a direction in which the scan line extendsintersect with a direction in which the signal line extends. It ispreferable that a direction in which a plurality of pixels exhibitingthe same color are aligned intersect with a direction in which thesignal line extends.

One embodiment of the present invention is a display module thatincludes a display device with one of the structures described above.The display module has a connector such as flexible printed circuit(FPC) board or a tape carrier package (TCP) connected thereto, or an ICis implemented on the display module with a method such as a chip onglass (COG) method or a chip on film (COF) method.

One embodiment of the present invention is an electronic deviceincluding the above-described display module and at least one of anantenna, a battery, a housing, a camera, a speaker, a microphone, and anoperation button.

One embodiment of the present invention can provide a liquid crystaldisplay device with high aperture ratio. One embodiment of the presentinvention can provide a liquid crystal display device with low powerconsumption. One embodiment of the present invention can provide ahigh-definition liquid crystal display device. One embodiment of thepresent invention can provide a highly reliable liquid crystal displaydevice.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily have all the effects. Other effects can be derived fromthe description of the specification, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an example of a displaydevice.

FIGS. 2A to 2C are cross-sectional views showing an example of a displaydevice.

FIGS. 3A and 3B are cross-sectional views showing examples of a displaydevice.

FIGS. 4A and 4B are top views showing an example of a subpixel.

FIGS. 5A and 5B are top views showing an example of a subpixel.

FIG. 6 is a cross-sectional view showing an example of a display device.

FIG. 7 is a cross-sectional view showing an example of a display device.

FIGS. 8A to 8D are cross-sectional view showing examples of a displaydevice.

FIGS. 9A and 9B show layout examples and structure examples of pixels.

FIGS. 10A and 10B are perspective views showing an example of a displaydevice.

FIGS. 11A and 11B are perspective views showing an example of a displaydevice.

FIGS. 12A to 12C show examples of an operation mode.

FIGS. 13A and 13B are a block diagram and a timing chart of a touchsensor.

FIGS. 14A and 14B are a block diagram and a timing chart of a displaydevice.

FIGS. 15A to 15D show the operations of a display portion and a touchsensor.

FIGS. 16A to 16D show the operations of a display portion and a touchsensor.

FIGS. 17A to 17C show examples of an electronic device.

FIGS. 18A to 18C show examples of an electronic device.

FIG. 19 is a cross-sectional view showing a display device of Example 1.

FIG. 20 shows the light transmittance of a layered structure included ina display device of Example 1.

FIGS. 21A1, 21A2, 21B1, 21B2, 21C1, and 21C2 show a manufacturing methodof a display device of Example 2.

FIGS. 22A1, 22A2, 22B1, 22B2, 22C1, 22C2, 22D1, and 22D2 show amanufacturing method of a display device of Example 2.

FIG. 23 shows the light transmittance of a layered structure included ina display device of Example 2.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the followingdescription. It will be readily appreciated by those skilled in the artthat modes and details of the present invention can be modified invarious ways without departing from the spirit and scope of the presentinvention. Thus, the present invention should not be construed as beinglimited to the description in the following embodiments.

Note that in structures of the present invention described below, thesame portions or portions having similar functions are denoted by thesame reference numerals in different drawings, and a description thereofis not repeated. Further, the same hatching pattern is applied toportions having similar functions, and the portions are not especiallydenoted by reference numerals in some cases.

The position, size, range, or the like of each structure illustrated indrawings is not accurately represented in some cases for easyunderstanding. Therefore, the disclosed invention is not necessarilylimited to the position, size, range, or the like disclosed in thedrawings.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film”. Also,the term “insulating film” can be changed into the term “insulatinglayer”.

In this specification and the like, a metal oxide means an oxide ofmetal in a broad sense. Metal oxides are classified into an oxideinsulator, an oxide conductor (including a transparent oxide conductor),an oxide semiconductor (also simply referred to as an OS), and the like.For example, a metal oxide used in a semiconductor layer of a transistoris called an oxide semiconductor in some cases. In other words, an OSFET is a transistor including a metal oxide or an oxide semiconductor.

In this specification and the like, a metal oxide including nitrogen isalso called a metal oxide in some cases. Moreover, a metal oxideincluding nitrogen may be called a metal oxynitride.

Embodiment 1

In this embodiment, a display device of one embodiment of the presentinvention is described with reference to FIG. 1, FIGS. 2A to 2C, FIGS.3A and 3B, FIGS. 4A and 4B, FIGS. 5A and 5B, FIG. 6, FIG. 7, FIGS. 8A to8D, FIGS. 9A and 9B, FIGS. 10A and 10B, and FIGS. 11A and 11B.

<1. Structure Example 1 of Display Device>

First, a display device of this embodiment is described with referenceto FIG. 1, FIGS. 2A to 2C, FIGS. 3A and 3B, FIGS. 4A and 4B, and FIGS.5A and 5B.

A display device of this embodiment includes a liquid crystal elementand a transistor. The liquid crystal element includes a pixel electrode,a liquid crystal layer, and a common electrode. The transistor iselectrically connected to the pixel electrode. A semiconductor layer ofthe transistor includes a stack of a first metal oxide layer and asecond metal oxide layer. The first metal oxide layer includes a regionwith lower crystallinity than the second metal oxide layer. Thetransistor includes a first region connected to the pixel electrode. Thepixel electrode, the common electrode, and the first region each have afunction of transmitting visible light. Visible light passes through thefirst region and the liquid crystal element and exits from the displaydevice.

A display device of this embodiment includes a liquid crystal elementand a transistor. The liquid crystal element includes a pixel electrode,a liquid crystal layer, and a common electrode. The transistor iselectrically connected to the pixel electrode. The transistor includes agate electrode, an insulating layer over the gate electrode, asemiconductor layer over the insulating layer, and a pair of electrodesover the semiconductor layer. The semiconductor layer includes a firstmetal oxide layer and a second metal oxide layer over the first metaloxide layer. The first metal oxide layer includes a region with lowercrystallinity than the second metal oxide layer. The transistor includesa first region connected to the pixel electrode. The pixel electrode,the common electrode, and the first region each have a function oftransmitting visible light. Visible light passes through the firstregion and the liquid crystal element and exits from the display device.

In the display device of this embodiment, a contact portion where thetransistor and the pixel electrode are in contact with each other can beprovided in a display region because the contact portion transmitsvisible light. Thus, the aperture ratio of the pixel can be increased.The higher the aperture ratio is, the more the light extractionefficiency can be increased. When the light extraction efficiency can beincreased, the luminance of a backlight unit can be decreased.Therefore, the power consumption of the display device can be reduced.Moreover, a high-definition display device can be obtained.

The display device of this embodiment further includes a scan line and asignal line. The scan line and the signal line are each electricallyconnected to the transistor. The scan line and the signal line eachinclude a metal layer. The scan line and the signal line each includingthe metal layer can have reduced resistance.

The scan line preferably includes a portion overlapping with a channelregion of the transistor. When a channel region of a transistor isirradiated with light, the characteristics of the transistor are changedin some cases depending on a material of the channel region of thetransistor. In the case where the portion of the scan line overlaps withthe channel region of the transistor, the irradiation of the channelregion with external light, light of a backlight, or the like can besuppressed. Thus, the reliability of the transistor can be improved. Oneconductive film may function as a scan line and a gate (or a back gate).

In one embodiment of the present invention, the transistor, a wiring, acapacitor, and the like can be formed using a light-transmittingsemiconductor material and a light-transmitting conductive materialdescribed below.

A semiconductor film in the transistor can be formed with alight-transmitting semiconductor material. Examples of thelight-transmitting semiconductor material include a metal oxide and anoxide semiconductor (OS). An oxide semiconductor preferably contains atleast indium (In). In particular, indium (In) and zinc (Zn) arepreferably contained. In addition, one or more of aluminum (Al), gallium(G), yttrium (Y), tin (Sn), copper, vanadium, beryllium, boron, silicon,titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum,cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the likemay be contained.

A conductive film in the transistor can be formed with alight-transmitting conductive material. The light-transmittingconductive material preferably contains one or more kinds of indium,zinc, and tin. Specifically, an In oxide, an In—Sn oxide (also referredto as an indium tin oxide or ITO), an In—Zn oxide, an In—W oxide, anIn—W—Zn oxide, an In—Ti oxide, an In—Sn—Ti oxide, an In—Sn—Si oxide, aZn oxide, a Ga—Zn oxide, or the like can be used.

The conductive film of the transistor may be an oxide semiconductor thatincludes an impurity element and has reduced resistance. The oxidesemiconductor with the reduced resistance can be regarded as an oxideconductor (OC).

For example, to form an oxide conductor, oxygen vacancies are formed inan oxide semiconductor and then hydrogen is added to the oxygenvacancies, so that a donor level is formed in the vicinity of theconduction band. The oxide semiconductor having the donor level has anincreased conductivity and becomes a conductor.

An oxide semiconductor has a large energy gap (e.g., an energy gap of2.5 eV or larger), and thus has a visible light transmitting property.An oxide conductor is an oxide semiconductor having a donor level in thevicinity of the conduction band as described above. Therefore, theinfluence of absorption due to the donor level is small in an oxideconductor, and an oxide conductor has a visible light transmittingproperty comparable to that of an oxide semiconductor.

The oxide conductor preferably includes one or more kinds of metalelements included in the semiconductor film of the transistor. When twoor more layers included in the transistor are formed using the oxidesemiconductors including the same metal element, the same manufacturingapparatus (e.g., deposition apparatus or processing apparatus) can beused in two or more steps and manufacturing cost can thus be reduced.

FIG. 1 is a perspective view of a display device 100A. For clarity,components such as a polarizer 130 are not drawn in FIG. 1. FIG. 1illustrates a substrate 61 with the dotted line. FIG. 2A and FIG. 3A arecross-sectional views of the display device 100A. FIG. 2B is an enlargedview of a transistor 201 included in the display device 100A. FIG. 2C isan enlarged view of a transistor 206 included in the display device100A. FIG. 3B is a modification example of the transistor 206 includedin the display device 100A.

The display device 100A includes a display portion 62 and a drivercircuit portion 64. An FPC 72 and an IC 73 are mounted on the displaydevice 100A.

The display portion 62 includes a plurality of pixels and has a functionof displaying images.

A pixel includes a plurality of subpixels. For example, the displayportion 62 can display a full-color image by having one pixel becomposed of three subpixels: a subpixel exhibiting a red color, asubpixel exhibiting a green color, and a subpixel exhibiting a bluecolor. Note that the color exhibited by subpixels is not limited to red,green, and blue. A pixel may be composed of subpixels that exhibitcolors of white, yellow, magenta, or cyan, for example. In thisspecification and the like, a subpixel may be simply described as apixel.

The display device 100A may include one or both of a scan line drivercircuit and a signal line driver circuit. The display device 100A mayinclude none of the scan line driver circuit and the signal line drivercircuit. When the display device 100A includes a sensor such as a touchsensor, the display device 100A may include a sensor driver circuit. Inthis embodiment, the driver circuit portion 64 is exemplified asincluding the scan line driver circuit. The scan line driver circuit hasa function of outputting a scan signal to the scan line included in thedisplay portion 62.

In the display device 100A, the IC 73 is mounted on a substrate 51 by aCOG method or the like. The IC 73 includes, for example, any one or moreof a signal line driver circuit, a scan line driver circuit, and asensor driver circuit.

The FPC 72 is electrically connected to the display device 100A. The IC73 and the driver circuit portion 64 are supplied with signals or powerfrom the outside through the FPC 72. Furthermore, signals can be outputto the outside from the IC 73 through the FPC 72.

An IC may be mounted on the FPC 72. For example, an IC including any oneor more of a signal line driver circuit, a scan line driver circuit, anda sensor driver circuit may be mounted on the FPC 72.

A wiring 65 supplies signals and power to the display portion 62 and thedriver circuit portion 64. The signals and power are input to the wiring65 from the outside through the FPC 72, or from the IC 73.

FIG. 2A and FIG. 3A are cross-sectional views including the displayportion 62, the driver circuit portion 64, and the wiring 65. In FIG. 2Aand the subsequent cross-sectional views of the display device, thedisplay portion 62 includes a display region 68 in a subpixel and anon-display region 66 around the display region 68.

In the example shown in FIG. 2A, the polarizer 130 is positioned on thesubstrate 61 side, and a backlight unit (not shown) is positioned on thesubstrate 51 side. Light 45 emitted from the backlight unit enters thesubstrate 51, passes through a contact portion where the transistor 206and a pixel electrode 111 are in contact with each other, a liquidcrystal element 40, a coloring layer 131, the substrate 61, and thepolarizer 130 in the order presented, and exits from the display device100A.

In the example shown in FIG. 3A, the polarizer 130 is positioned on thesubstrate 51 side, and the backlight unit (not shown) is positioned onthe substrate 61 side. The light 45 emitted from the backlight unitenters the substrate 61, passes through the coloring layer 131, theliquid crystal element 40, the contact portion where the substrate 206and the pixel electrode 111 are in contact with each other, thesubstrate 51, and the polarizer 130 in the order presented, and exitsfrom the display device 100A.

As described above, in the display device of this embodiment, either asurface on the substrate 51 side or a surface on the substrate 61 sidecan be used as the display surface without changing the structureexisting between the substrate 51 and the substrate 61. Thedetermination of the display surface can be made as appropriatedepending on the position of the backlight unit, the polarizer, thetouch sensor, or the like.

Although the following description is made using FIG. 2A as an example,the following description can also apply to FIG. 3A.

The display device 100A is an example of a transmissive liquid crystaldisplay device that includes a liquid crystal element with a horizontalelectric field mode.

As illustrated in FIG. 2A, the display device 100A includes thesubstrate 51, the transistor 201, the transistor 206, the liquid crystalelement 40, an alignment film 133 a, an alignment film 133 b, aconnection portion 204, an adhesive layer 141, the coloring layer 131, alight-blocking layer 132, an overcoat 121, the substrate 61, thepolarizer 130, and the like.

The transistor 206 is provided in the non-display region 66. FIG. 2C isan enlarged view of the transistor 206.

The transistor 206 includes a gate 221, an insulating layer 213, aconductive layer 222 a, a conductive layer 222 c, and a semiconductorlayer 231.

The gate 221 overlaps with the semiconductor layer 231 with theinsulating layer 213 positioned therebetween. The insulating layer 213functions as a gate insulating layer. Each of the conductive layers 222a and 222 c is connected to the semiconductor layer 231.

In FIG. 2A, the pixel electrode 111 included in the liquid crystalelement 40 is electrically connected to the semiconductor layer 231 withthe conductive layer 222 c positioned therebetween.

The conductive layer 222 c is formed using a material that transmitsvisible light. Thus, the contact portion where the pixel electrode 111and the transistor are in contact with each other can be provided in thedisplay region 68. Accordingly, the aperture ratio of the subpixel canbe increased and the power consumption of the display device can bereduced.

As shown in FIG. 2C, the semiconductor layer 231 includes a first metaloxide layer 231 a and a second metal oxide layer 231 b over the firstmetal oxide layer 231 a.

The first metal oxide layer 231 a and the second metal oxide layer 231 beach preferably include In, M (M is Ga, Al, Y, or Sn), and Zn.

It is preferable that the first metal oxide layer 231 a and the secondmetal oxide layer 231 b each have a region where the atomic proportionof In is higher than the atomic proportion of M because the field-effectmobility of the transistor is increased in such a structure. Forexample, the atomic ratio of In to M and Zn in each of the first metaloxide layer 231 a and the second metal oxide layer 231 b is preferablyIn:M:Zn=4:2:3 or a neighborhood of In:M:Zn=4:2:3, or In:M:Zn=5:1:7 or aneighborhood of In:M:Zn=5:1:7. The term “neighborhood” includes thefollowing: when In is 4, M is greater than or equal to 1.5 and less thanor equal to 2.5, and Zn is greater than or equal to 2 and less than orequal to 4. Also the term “neighborhood” includes the following: when Inis 5, M is greater than or equal to 0.5 and less than or equal to 1.5,and Zn is greater than or equal to 5 and less than or equal to 7. Whenthe compositions of the first metal oxide layer 231 a and the secondmetal oxide layer 231 b are substantially the same, they can be formedusing the same sputtering target and the manufacturing cost can thus bereduced.

For the first metal oxide layer 231 a and the second metal oxide layer231 b, it is particularly preferable to use stacked films depositedsuccessively without exposure to the air using targets with the samecomposition, although it is also possible to use films deposited usingtargets with different compositions. When the films are depositedsuccessively, one deposition apparatus can be shared between a pluralityof deposition steps, and remaining of impurities between the first metaloxide layer 231 a and the second metal oxide layer 231 b can besuppressed.

It is preferable that the second metal oxide layer 231 b include aregion having higher crystallinity than the first metal oxide layer 231a. Including such a high-crystallinity region, the second metal oxidelayer 231 b can have higher etching resistance than the first metaloxide layer 231 a. Thus, it is possible to prevent the removal of thesecond metal oxide layer 231 b due to etching when the conductive layer222 a and the conductive layer 222 c are processed. As a result, achannel-etched transistor as illustrated in FIGS. 2A and 2B can beformed. Furthermore, when a high-crystallinity film is used for thesecond metal oxide layer 231 b positioned on the back channel side ofthe transistor, the amount of impurities which may diffuse into thefirst metal oxide layer 231 a positioned on the gate 221 side can bereduced. Thus, a transistor with high reliability can be obtained.

Furthermore, when the first metal oxide layer 231 a includes a filmincluding a region having lower crystallinity than the second metaloxide layer 231 b, oxygen easily diffuses into the first metal oxidelayer 231 a, and the proportion of oxygen vacancy in the first metaloxide layer 231 a can be reduced. In particular, the first metal oxidelayer 231 a is positioned close to the gate 221 and is a main layerwhere a channel is easily formed. Thus, when such a film is used for thefirst metal oxide layer 231 a, a highly reliable transistor can beobtained.

The first metal oxide layer 231 a and the second metal oxide layer 231 bcan be formed in different manners, for example, with differentdeposition conditions. For example, the oxygen gas flow rate in thedeposition gas can be varied between the first metal oxide layer 231 aand the second metal oxide layer 231 b.

In this case, as the deposition condition of the first metal oxide layer231 a, the proportion of oxygen gas flow rate (also referred to asoxygen flow rate ratio) in a whole deposition gas is higher than orequal to 0% and lower than or equal to 30%, preferably higher than orequal to 5% and lower than or equal to 15%. With the above oxygen flowrate ratio, the first metal oxide layer 231 a can have lowcrystallinity.

As the deposition condition of each of the second metal oxide layer 231b, the oxygen flow rate ratio is higher than 30% and lower than or equalto 100%, preferably higher than or equal to 50% and lower than or equalto 100%, further preferably higher than or equal to 70% and lower thanor equal to 100%. With the above oxygen flow rate ratio, the secondmetal oxide layer 231 b can have high crystallinity.

The substrate temperature at which the first metal oxide layer 231 a andthe second metal oxide layer 231 b are formed is preferably higher thanor equal to room temperature (25° C.) and lower than or equal to 200°C., further preferably higher than or equal to room temperature andlower than or equal to 130° C. The substrate temperature in the aboverange can prevent bending or warpage of the substrate in the case wherethe substrate is a large glass substrate. In the case where the firstmetal oxide layer 231 a and the second metal oxide layer 231 b areformed with the same substrate temperature, the productivity can beincreased. In the case where the first metal oxide layer 231 a and thesecond metal oxide layer 231 b are formed with different substratetemperatures, for example, the substrate temperature in forming thesecond metal oxide layer 231 b is higher than that in forming the firstmetal oxide layer 231 a, the crystallinity of the second metal oxidelayer 231 b can be further increased.

For example, it is preferable that a cloud-aligned composite oxidesemiconductor (CAC-OS) film be used for the first metal oxide layer 231a and a c-axis-aligned crystalline oxide semiconductor (CAAC-OS) film beused for the second metal oxide layer 231 b.

A conductive layer used as the gate 221 may also function as a scanline. That is, one conductive layer may function as a scan line and thegate 221. A conductive layer used as the conductive layer 222 a may alsofunction as a signal line. That is, one conductive layer may function asa signal line and the conductive layer 222 a. It is preferable that theresistance of the conductive layer functioning as a scan line or asignal line be sufficiently low. Therefore, it is preferable that theconductive layer functioning as a scan line or a signal line be formedusing a metal, an alloy, or the like. The conductive layer functioningas a scan line or a signal line may be formed using a material having afunction of blocking visible light.

Specifically, in some cases, a conductive material that transmitsvisible light has higher resistivity than a conductive material thatblocks visible light, such as copper or aluminum. Thus, a conductivematerial (a metal material) that has low resistivity and blocks visiblelight is preferably used as a bus line such as a scan line or a signalline to prevent signal delay. Note that a conductive material thattransmits visible light can be used for a bus line depending on the sizeof the pixel, the width of the bus line, the thickness of the bus line,or the like.

When the gate 221 is formed using a conductive layer that blocks visiblelight, irradiation of the semiconductor layer 231 with light emittedfrom the backlight can be suppressed. When a conductive layer thatblocks visible light overlaps with the semiconductor layer 231 in thismanner, the variation in the characteristics of the transistor due tolight can be suppressed. Accordingly, the reliability of the transistorcan be improved.

The light-blocking layer 132 is provided between the semiconductor layer231 and the substrate 61, and the gate 221 that blocks visible light isprovided between the semiconductor layer 231 and the substrate 51. Thisstructure can suppress irradiation of the semiconductor layer 231 withexternal light and light emitted from the backlight.

The modification example of the transistor 206 is shown in FIG. 3B. InFIG. 3B, the semiconductor layer 231 of the transistor 206 is partlypositioned in the display region 68. In the case where the semiconductorlayer of the transistor is formed using silicon, typically amorphoussilicon or low-temperature polysilicon, the semiconductor layer absorbspart of visible light; accordingly, it is difficult to extract lightthrough the semiconductor layer. When an impurity such as phosphorus orboron is included in silicon, the light transmitting property is moredecreased in some cases, and accordingly, it is more difficult toextract light through a low-resistance region formed in silicon in somecases. However, in one embodiment of the present invention, an oxidesemiconductor (OS) and an oxide conductor (OC) each have a visible lighttransmitting property, so that the aperture ratio of the pixel or thesubpixel can be increased.

The transistor 206 is covered by an insulating layer 212, an insulatinglayer 214, and an insulating layer 215. Note that the insulating layers212 and 214 can be considered as the components of the transistor 206.The transistor is preferably covered by an insulating layer that reducesthe diffusion of an impurity to the semiconductor constituting thetransistor. The insulating layer 215 can function as a planarizationlayer.

Each of the insulating layers 212 and 213 preferably includes an excessoxygen region. When each of the insulating layers 212 and 213 includesan excess oxygen region, excess oxygen can be supplied to thesemiconductor layer 231. A highly reliable transistor can be providedsince oxygen vacancies that are potentially formed in the semiconductorlayer 231 can be filled with excess oxygen.

It is preferable to use an oxide insulating film formed in anoxygen-containing atmosphere, such as a silicon oxide film or a siliconoxynitride film, for the insulating layer 212. An insulating film withlow oxygen diffusibility and oxygen permeability, such as a siliconnitride film, is preferably stacked as the insulating layer 214 over thesilicon oxide film or the silicon oxynitride film. The oxide insulatingfilm formed in an atmosphere containing oxygen can easily release alarge amount of oxygen by heating. When a stack including such an oxideinsulating film that releases oxygen and an insulating film with lowoxygen diffusibility and oxygen permeability is heated, oxygen can besupplied to the semiconductor layer 231. As a result, oxygen vacanciesin the semiconductor layer 231 can be filled and defects at theinterface between the semiconductor layer 231 and the insulating layer212 can be repaired, leading to a reduction in the density of defectstates. Accordingly, an extremely highly reliable display device can befabricated.

The liquid crystal element 40 is provided in the display region 68. Theliquid crystal element 40 is a liquid crystal element using a fringefield switching (FFS) mode.

The liquid crystal element 40 includes the pixel electrode 111, a commonelectrode 112, and a liquid crystal layer 113. The alignment of theliquid crystal layer 113 can be controlled with the electrical fieldgenerated between the pixel electrode 111 and the common electrode 112.The liquid crystal layer 113 is positioned between the alignment films133 a and 133 b.

The common electrode 112 may have a top-surface shape (also referred toas a planar shape) that has a comb-like shape or a top-surface shapethat is provided with a slit. FIG. 2A and FIG. 3A illustrate an examplein which one opening is provided in the common electrode 112 in thedisplay region 68 of one subpixel. One opening or a plurality ofopenings can be provided in the common electrode 112. As the definitionof the display device increases, the area of the display region 68 inone subpixel becomes smaller. Thus, the number of openings provided inthe common electrode 112 is not limited to more than one; one openingcan be provided. That is, in a high-definition display device, the areaof the pixel (the subpixel) is small; therefore, an adequate electricfield for the alignment of liquid crystals over the entire displayregion of the subpixel can be generated, even when there is only oneopening in the common electrode 112.

An insulating layer 220 is provided between the pixel electrode 111 andthe common electrode 112. The pixel electrode 111 includes a portionthat overlaps with the common electrode 112 with the insulating layer220 provided therebetween. Furthermore, the common electrode 112 is notplaced over the pixel electrode 111 in some areas of a region where thepixel electrode 111 and the coloring layer 131 overlap.

An alignment film is preferably provided in contact with the liquidcrystal layer 113. The alignment film can control the alignment of theliquid crystal layer 113. In the display device 100A, the alignment film133 a is positioned between the common electrode 112 (or the insulatinglayer 220) and the liquid crystal layer 113, and the alignment film 133b is positioned between the overcoat 121 and the liquid crystal layer113.

The liquid crystal material is classified into a positive liquid crystalmaterial with a positive dielectric anisotropy (Ac) and a negativeliquid crystal material with a negative dielectric anisotropy. Both ofthe materials can be used in one embodiment of the present invention,and an optimal liquid crystal material can be selected according to theemployed mode and design.

In one embodiment of the present invention, a negative liquid crystalmaterial is preferably used. The negative liquid crystal is lessaffected by a flexoelectric effect and thus the polarity of voltageapplied to the liquid crystal layer makes little difference in lighttransmittance. This prevents flickering from being recognized by theuser of the display device. The flexoelectric effect is a phenomenon inwhich polarization is induced by the distortion of orientation, andmainly depends on the shape of a molecule. The negative liquid crystalmaterial is less likely to experience the deformation such as spreadingand bending.

Note that the liquid crystal element 40 is an element using an FFS modehere; however, one embodiment of the present invention is not limitedthereto, and a liquid crystal element using any of a variety of modescan be used. For example, a liquid crystal element using a verticalalignment (VA) mode, a twisted nematic (TN) mode, an in-plane switching(IPS) mode, an axially symmetric aligned micro-cell (ASM) mode, anoptically compensated birefringence (OCB) mode, a ferroelectric liquidcrystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, anelectrically controlled birefringence (ECB) mode, a VA-IPS mode, aguest-host mode, or the like can be used.

Furthermore, the display device 100A may be a normally black liquidcrystal display device, for example, a transmissive liquid crystaldisplay device using a vertical alignment (VA) mode. Examples of thevertical alignment mode include a multi-domain vertical alignment (MVA)mode, a patterned vertical alignment (PVA) mode, and an advanced superview (ASV) mode.

Note that the liquid crystal element is an element that controlstransmission and non-transmission of light by optical modulation actionof the liquid crystal. Optical modulation action of the liquid crystalis controlled by an electric field applied to the liquid crystal(including a horizontal electric field, a vertical electric field, or anoblique electric field). As the liquid crystal used for the liquidcrystal element, a thermotropic liquid crystal, a low-molecular liquidcrystal, a high-molecular liquid crystal, a polymer dispersed liquidcrystal (PDLC), a ferroelectric liquid crystal, an anti-ferroelectricliquid crystal, or the like can be used. Such a liquid crystal materialexhibits a cholesteric phase, a smectic phase, a cubic phase, a chiralnematic phase, an isotropic phase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric fieldmode, a liquid crystal exhibiting a blue phase for which an alignmentfilm is unnecessary may be used. A blue phase is one of liquid crystalphases, which is generated just before a cholesteric phase changes intoan isotropic phase while temperature of cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which 5 wt. % or more of a chiralmaterial is mixed is preferably used for the liquid crystal layer 113 inorder to improve the temperature range. The liquid crystal compositionwhich includes a liquid crystal exhibiting a blue phase and a chiralmaterial has a short response time and exhibits optical isotropy, whichmakes the alignment process unnecessary. In addition, the liquid crystalcomposition which includes liquid crystal exhibiting a blue phase and achiral material has a small viewing angle dependence. In addition, sincean alignment film does not need to be provided and rubbing treatment isunnecessary, electrostatic discharge damage caused by the rubbingtreatment can be prevented and defects or damage of the liquid crystaldisplay device in the manufacturing process can be reduced.

As the display device 100A is a transmissive liquid crystal displaydevice, a conductive material that transmits visible light is used forboth the pixel electrode 111 and the common electrode 112. A conductivematerial that transmits visible light is used for one or more of theconductive layers included in the transistor 206. Accordingly, at leasta part of the transistor 206 can be provided in the display region 68.FIGS. 2A and 2B show the case where a semiconductor material thattransmits visible light is used for the conductive layer 222 c.

For example, a material containing one or more of indium (In), zinc(Zn), and tin (Sn) is preferably used for the conductive material thattransmits visible light. Specifically, indium oxide, indium tin oxide(ITO), indium zinc oxide, indium oxide containing tungsten oxide, indiumzinc oxide containing tungsten oxide, indium oxide containing titaniumoxide, indium tin oxide containing titanium oxide, indium tin oxidecontaining silicon oxide (ITSO), zinc oxide, and zinc oxide containinggallium are given, for example. Note that a film including graphene canbe used as well. The film including graphene can be formed, for example,by reducing a film including graphene oxide.

An oxide conductive layer is preferably used for one or more of theconductive layer 222 c, the pixel electrode 111, and the commonelectrode 112. The oxide conductive layer preferably includes one ormore metal elements that are included in the semiconductor layer 231 ofthe transistor 206. For example, the conductive layer 222 c preferablyincludes indium and is further preferably an oxide film including In, M,and Zn (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, Sn, or Hf). Similarly, eachof the pixel electrode 111 and the common electrode 112 preferablyincludes indium and is further preferably an oxide film including In, M,and Zn (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, Sn, or Hf).

An oxide semiconductor may be used for one or more of the conductivelayer 222 c, the pixel electrode 111, and the common electrode 112. Whentwo or more layers constituting the display device are formed usingoxide semiconductors containing the same metal element, the samemanufacturing equipment (e.g., film-formation equipment or processingequipment) can be used in two or more steps; manufacturing cost can thusbe reduced.

An oxide semiconductor is a semiconductor material whose resistance canbe controlled by oxygen vacancies in the film of the semiconductormaterial and/or the concentration of impurities such as hydrogen orwater in the film of the semiconductor material. Thus, the resistivityof the oxide conductive layer can be controlled by selecting treatmentfor increasing oxygen vacancies and/or impurity concentration on theoxide semiconductor layer, or treatment for reducing oxygen vacanciesand/or impurity concentration on the oxide semiconductor layer.

Note that such an oxide conductive layer formed using an oxidesemiconductor layer can be referred to as an oxide semiconductor layerhaving a high carrier density and a low resistance, an oxidesemiconductor layer having conductivity, or an oxide semiconductor layerhaving high conductivity.

In addition, the manufacturing cost can be reduced by forming the oxidesemiconductor layer and the oxide conductive layer using the same metalelement. For example, the manufacturing cost can be reduced by using ametal oxide target with the same metal composition. By using the metaloxide target with the same metal composition, an etching gas or anetchant used in the processing of the oxide semiconductor layer can alsobe used for processing of the oxide conductive layer. Note that evenwhen the oxide semiconductor layer and the oxide conductive layer havethe same metal elements, they have different compositions in some cases.For example, a metal element in the film is released during themanufacturing process of the display device, which results in adifferent metal composition.

For example, when a silicon nitride film containing hydrogen is used forthe insulating layer 212 and an oxide semiconductor is used for theconductive layer 222 c, the conductivity of the oxide semiconductor canbe increased by the hydrogen that is supplied from the insulating layer212. For example, when a silicon nitride film containing hydrogen isused for the insulating layer 220 and an oxide semiconductor is used forthe pixel electrode 111, the conductivity of the oxide semiconductor canbe increased by the hydrogen that is supplied from the insulating layer220.

In the display device 100A, the coloring layer 131 and thelight-blocking layer 132 are provided closer to the substrate 61 thanthe liquid crystal layer 113 is. The coloring layer 131 is positioned ina region that at least overlaps with the display region 68 of asubpixel. In the non-display region 66 of a pixel (subpixel), thelight-blocking layer 132 is provided. The light-blocking layer 132overlaps with at least a part of the transistor 206.

The overcoat 121 is preferably provided between the coloring layer 131or the light-blocking layer 132, and the liquid crystal layer 113. Theovercoat 121 can reduce the diffusion of an impurity contained in thecoloring layer 131 and the light-blocking layer 132 and the like intothe liquid crystal layer 113.

The substrates 51 and 61 are bonded to each other by the adhesive layer141. The liquid crystal layer 113 is encapsulated in a region that issurrounded by the substrates 51 and 61, and the adhesive layer 141.

When the display device 100A functions as a transmissive liquid crystaldisplay device, two polarizers are positioned in a way that the displayportion 62 is sandwiched between the two polarizers. FIG. 2A illustratesthe polarizer 130 on the substrate 61 side. The light 45 from abacklight provided on the outside of the polarizer on the substrate 51side enters the display device 100A through the polarizer. In this case,the optical modulation of the light can be controlled by controlling thealignment of the liquid crystal layer 113 with a voltage suppliedbetween the pixel electrode 111 and the common electrode 112. That is,the intensity of light ejected through the polarizer 130 can becontrolled. Furthermore, the coloring layer 131 absorbs light ofwavelengths other than a specific wavelength range from the incidentlight. As a result, the ejected light is light that exhibits red, blue,or green colors, for example.

In addition to the polarizer, a circular polarizer can be used, forexample. An example of a circular polarizer is a stack including alinear polarizer and a quarter-wave retardation plate. The circularpolarizer can reduce the viewing angle dependence of the display qualityof the display device.

The driver circuit portion 64 includes the transistor 201. FIG. 2B is anenlarged view of the transistor 201.

The transistor 201 includes the gate 221, the insulating layer 213, thesemiconductor layer 231, the conductive layer 222 a, and the conductivelayer 222 b. One of the conductive layers 222 a and 222 b functions as asource, and the other functions as a drain. The conductive layer 222 aand the conductive layer 222 b are electrically connected to thesemiconductor layer 231.

As shown in FIG. 2B, the semiconductor layer 231 includes the firstmetal oxide layer 231 a and the second metal oxide layer 231 b over thefirst metal oxide layer 231 a. The description of the transistor 206 canbe referred to for the details of the semiconductor layer 231.

The transistor provided in the driver circuit portion 64 does notnecessarily have a function of transmitting visible light. Thus, theconductive layer 222 a and the conductive layer 222 b can be formedusing the same manufacturing step and the same material (preferably, amaterial with low resistivity such as metal).

In the connection portion 204, the wiring 65 and a conductive layer 251are connected to each other, and the conductive layer 251 and aconnector 242 are connected to each other. That is, in the connectionportion 204, the wiring 65 is electrically connected to the FPC 72through the conductive layer 251 and the connector 242. By employingthis configuration, signals and power can be supplied from the FPC 72 tothe wiring 65.

The wiring 65 can be formed using the same material and the samemanufacturing step as those used for forming the conductive layer 222 aand 222 b included in the transistor 201 and the conductive layer 222 aincluded in the transistor 206. The conductive layer 251 can be formedusing the same material and the same manufacturing step as those usedfor forming the pixel electrode 111 included in the liquid crystalelement 40. Forming the conductive layers constituting the connectionportion 204 in such a manner, i.e., using the same materials and thesame manufacturing steps as those used for forming the conductive layerscomposing the display portion 62 and the driver circuit portion 64, ispreferable because this can reduce the number of process steps.

The transistors 201 and 206 may have the same structure or differentstructures. That is, the transistor included in the driver circuitportion 64 and the transistor included in the display portion 62 mayhave the same structure or different structures. In addition, the drivercircuit portion 64 may have a plurality of transistors with differentstructures, and the display portion 62 may have a plurality oftransistors with different structures. For example, a transistor havinga structure in which two gates are electrically connected to each otheris preferably used for one or more of a shift register circuit, a buffercircuit, and a protection circuit included in the scan line drivercircuit.

[Structure Example of Subpixel]

FIGS. 4A and 4B are top views of subpixels of one embodiment of thepresent invention. FIGS. 5A and 5B are top views of comparisonsubpixels.

First, characteristics of a pixel (a subpixel) of one embodiment of thepresent invention are described, though some of them have been describedabove.

A pixel includes a transistor, a capacitor, a scan line, a signal line,and the like. In general, these components are formed using metal filmswith low resistivity. Since a metal film does not transmit light,portions formed using metal films are excluded from a display region.This decreases the aperture ratio of a pixel. In particular, when thedefinition increases, the aperture ratio decreases significantly. In thecase where the aperture ratio decreases in a liquid crystal displaydevice, a backlight needs to emit a larger amount of light to increasethe luminance and the contrast, leading to higher power consumption ofthe backlight.

In one embodiment of the present invention, a structure that transmitsvisible light is used in one or more of the transistor, the capacitor,the wiring, and the contact portion in the pixel. Specifically, amaterial that transmits visible light, such as an oxide semiconductor oran oxide conductor, is used in these components. These componentsprovided in the pixel transmit visible light, so that the aperture ratiocan be increased and the power consumption of the backlight can bedecreased. Note that the scan line, the signal light, a power supplyline, and a peripheral circuit are each formed of a metal material todecrease the resistance. As thus described, conductive films arepreferably formed separately by selecting material depending on thefunctions of the components.

Using a material that transmits visible light, such as an oxidesemiconductor or an oxide conductor, enables formation of transistorswith a variety of structures. Unlike silicon, an oxide semiconductor hasa visible light transmitting property even when the oxide semiconductorhas reduced resistance by being doped with an impurity.

FIGS. 4A and 4B and FIGS. 5A and 5B are top views of subpixels eachincluding a liquid crystal element with a vertical electric field modesuch as a TN mode or a VA mode. FIGS. 4A and 4B are top views ofsubpixels of one embodiment of the present invention. FIGS. 5A and 5Bare top views of comparison subpixels.

FIGS. 4A and 5A are top views of a layered structure from a gate 223 tothe pixel electrode 111 in the subpixel that is seen from the pixelelectrode 111 side. In FIGS. 4A and 5A, the display region 68 in thesubpixel is outlined in a bold dotted line. FIGS. 4B and 5B are topviews each obtained by excluding the pixel electrode 111 from thelayered structure shown in FIG. 4A or FIG. 5A.

In each of the transistors shown in FIGS. 4A and 4B and FIGS. 5A and 5B,gates are provided over and under a channel.

The gate 221 and the gate 223 are electrically connected to each other.Transistors having such a structure in which two gates are electricallyconnected to each other can have a higher field-effect mobility and thushave higher on-state current than other transistors. Consequently, acircuit capable of high-speed operation can be obtained. Furthermore,the area occupied by a circuit portion can be reduced. The use of thetransistor having a high on-state current can reduce signal delay inwirings and can suppress display unevenness even in a display device inwhich the number of wirings is increased because of an increase in sizeor definition. Moreover, with such a structure, a highly reliabletransistor can be formed.

In other words, in FIGS. 4A and 4B and FIGS. 5A and 5B, one conductivelayer functions as a scan line 228 and the gate 223. One of the gates221 and 223 that has the lower resistance of the two is preferably theconductive layer that also functions as the scan line. It is preferablethat the resistance of the conductive layer functioning as the scan line228 be sufficiently low. Therefore, it is preferable that the conductivelayer functioning as the scan line 228 be formed using a metal, analloy, or the like. The conductive layer functioning as the scan line228 may be formed using a material having a function of blocking visiblelight.

In FIGS. 4A and 4B and FIGS. 5A and 5B, one conductive layer functionsas a signal line 229 and the conductive layer 222 a. It is preferablethat the resistance of the conductive layer functioning as the signalline 229 be sufficiently low. Therefore, it is preferable that theconductive layer functioning as the signal line 229 be formed using ametal, an alloy, or the like. The conductive layer functioning as thesignal line 229 may be formed using a material having a function ofblocking visible light.

The gates 221 and 223 can each include a single layer of one of a metalmaterial and an oxide conductor, or stacked layers of both a metalmaterial and an oxide conductor. For example, one of the gates 221 and223 may include an oxide conductor, and the other of the gates 221 and223 may include a metal material.

The transistor can include an oxide semiconductor layer as thesemiconductor layer, and include an oxide conductive layer as at leastone of the gates 221 and 223. In this case, the oxide semiconductorlayer and the oxide conductive layer are preferably formed using anoxide semiconductor.

FIGS. 4A and 4B and FIGS. 5A and 5B show an example of providing acapacitor line 244 in the subpixel. The capacitor line 244 iselectrically connected to a conductive layer formed using the samematerial and the same manufacturing step as those used for forming theconductive layer (e.g., the gate 221) included in the transistor. InFIGS. 4A and 4B, the conductive layer 222 c that overlaps with thecapacitor line 244 and transmits visible light is provided. In FIGS. 5Aand 5B, the conductive layer 222 b that overlaps with the capacitor line244 and blocks visible light is provided. In FIGS. 4A and 4B, theconductive layer 222 c is connected to the pixel electrode 111. In FIGS.5A and 5B, the conductive layer 222 b is connected to the pixelelectrode 111.

In the structure shown in FIGS. 4A and 4B, the contact portion where theconductive layer 222 c and the pixel electrode 111 are in contact witheach other and at least part of the capacitor can be provided in thedisplay region 68. Thus, the subpixel with the structure shown in FIGS.4A and 4B can have a higher aperture ratio than the subpixel with thestructure shown in FIGS. 5A and 5B. Moreover, the power consumption ofthe display device can be reduced.

When the contact portion where the pixel electrode 111 and thetransistor are in contact with each other and the capacitor are providedin the display region 68 in one embodiment of the present invention, theaperture ratio can be increased by 10% or more, or 20% or more.Accordingly, the power consumption of the backlight can be decreased by10% or more, or 20% or more.

The degree of change in the aperture ratio and the power consumption ofthe backlight due to change in the structure from that shown in FIGS. 5Aand 5B to that shown in FIGS. 4A and 4B is estimated below.

Described here is the case where a display for a large TV is assumed andthe layouts of the subpixels shown in FIGS. 4A and 4B and FIGS. 5A and5B are used in TN mode liquid crystal display devices in which the pixeldensity is 136 ppi, the size of the display region is 65 inchesdiagonally, and the resolution is 8K.

The size of the subpixel is 62.5 μm×187.5 μm. The liquid crystal elementhas a vertical electric field mode. The storage capacitor can be formedbetween a gate wiring and a source wiring or a drain wiring. Since120-Hz driving is assumed, two signal lines are provided in onesubpixel. The transistor has a BGTC channel-etched structure.

The aperture ratio of the pixel layout shown in FIG. 5A is 37.3%. Theaperture ratio of the pixel layout shown in FIG. 4A is 47.1%. By using avisible light transmitting structure for the contact portion where thetransistor and the pixel electrode are in contact with each other andthe storage capacitor, the aperture ratio can be 1.26 times as large asthe aperture ratio of FIG. 5A. It is estimated that the powerconsumption of the backlight can be decreased by approximately 21%.

[Materials]

Next, the details of the materials that can be used for components ofthe display device of this embodiment and the like are described. Notethat description on the components already described is omitted in somecases. The materials described below can be used as appropriate in thedisplay device, a touch panel, and the components thereof describedlater.

<<Substrates 51 and 61>>

There are no large limitations on the material of the substrate used inthe display device of one embodiment of the present invention; a varietyof substrates can be used. For example, a glass substrate, a quartzsubstrate, a sapphire substrate, a semiconductor substrate, a ceramicsubstrate, a metal substrate, a plastic substrate or the like can beused.

The weight and thickness of the display device can be reduced by using athin substrate. Furthermore, a flexible display device can be obtainedby using a substrate that is thin enough to have flexibility.

The display device of one embodiment of the present invention isfabricated by forming a transistor and the like over a fabricationsubstrate, then transferring the transistor and the like on anothersubstrate. The use of the fabrication substrate enables the following: aformation of a transistor with favorable characteristics; a formation ofa transistor with low power consumption; a manufacturing of a durabledisplay device, an addition of heat resistance to the display device, amanufacturing of a more lightweight display device, or a manufacturingof a thinner display device. Examples of a substrate to which atransistor is transferred include, in addition to the substrate overwhich the transistor can be formed, a paper substrate, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), and the like),a leather substrate, a rubber substrate, and the like.

<<Transistors 201 and 206>>

A transistor included in the display device of one embodiment of thepresent invention may have a top-gate structure or a bottom-gatestructure. Gate electrodes may be provided above and below a channel.There is no particular limitation on a semiconductor material used forthe transistor, and an oxide semiconductor, silicon, or germanium can beused, for example.

There is no particular limitation on the crystallinity of asemiconductor material used for the transistor, and an amorphoussemiconductor or a semiconductor having crystallinity (amicrocrystalline semiconductor, a polycrystalline semiconductor, asingle-crystal semiconductor, or a semiconductor partly includingcrystal regions) may be used. A semiconductor having crystallinity ispreferably used, in which case deterioration of the transistorcharacteristics can be suppressed.

For example, a Group 14 element, a compound semiconductor, or an oxidesemiconductor can be used for the semiconductor layer. Typically, asemiconductor including silicon, a semiconductor including galliumarsenide, or an oxide semiconductor including indium can be used for thesemiconductor layer.

An oxide semiconductor is preferably used as a semiconductor in which achannel of the transistor is formed. In particular, an oxidesemiconductor having a wider band gap than silicon is preferably used.The use of a semiconductor material with a larger bandgap than siliconand a small carrier density is preferable because the current during theoff state (off-state current) of the transistor can be reduced.

For the oxide semiconductor, the above description and Embodiment 4 canbe referred to, for example.

The use of such an oxide semiconductor achieves a highly reliabletransistor with little change in the electrical characteristics.

Charge accumulated in the capacitor through the transistor can beretained for a long time because of low off-state current of thetransistor. The use of such a transistor in pixels allows a drivercircuit to stop while the gray level of a displayed image is maintained.As a result, a display device with extremely low power consumption isobtained.

The transistors 201 and 206 preferably include an oxide semiconductorlayer that is highly purified to reduce the formation of oxygenvacancies. Accordingly, the off-state current of the transistors can bemade small. Accordingly, an electrical signal such as an image signalcan be held for a longer period, and a writing interval can be setlonger in an on state. Accordingly, the frequency of refresh operationcan be reduced, which leads to an effect of suppressing powerconsumption.

In the transistors 201 and 206, relatively high field-effect mobilitycan be obtained, whereby high-speed operation is possible. The use ofsuch transistors that are capable of high-speed operation in the displaydevice enables the fabrication of the transistor in the display portionand the transistor in the driver circuit portion over the samesubstrate. This means that a semiconductor device separately formed witha silicon wafer or the like does not need to be used as the drivercircuit, which enables a reduction of the number of components in thedisplay device. In addition, the transistor that can operate at highspeed can be used also in the display portion, whereby a high-qualityimage can be provided.

<<Insulating Layer>>

An organic insulating material or an inorganic insulating material canbe used as an insulating material that can be used for the insulatinglayer, the overcoat, the spacer, or the like included in the displaydevice. Examples of an organic insulating material include an acrylicresin, an epoxy resin, a polyimide resin, a polyamide resin, apolyamide-imide resin, a siloxane resin, a benzocyclobutene-based resin,and a phenol resin. Examples of an inorganic insulating layer include asilicon oxide film, a silicon oxynitride film, a silicon nitride oxidefilm, a silicon nitride film, an aluminum oxide film, a hafnium oxidefilm, an yttrium oxide film, a zirconium oxide film, a gallium oxidefilm, a tantalum oxide film, a magnesium oxide film, a lanthanum oxidefilm, a cerium oxide film, and a neodymium oxide film.

<<Conductive Layer>>

For the conductive layer such as the gate, the source, and the drain ofthe transistor and the wiring and the electrode of the display device, asingle-layer structure or a layered structure using any of metals suchas aluminum, titanium, chromium, nickel, copper, yttrium, zirconium,molybdenum, silver, tantalum, and tungsten, or an alloy containing anyof these metals as its main component can be used. For example, atwo-layer structure in which a titanium film is stacked over an aluminumfilm; a two-layer structure in which a titanium film is stacked over atungsten film; a two-layer structure in which a copper film is stackedover a molybdenum film; a two-layer structure in which a copper film isstacked over an alloy film containing molybdenum and tungsten; atwo-layer structure in which a copper film is stacked over an alloy filmcontaining copper, magnesium, and aluminum; a three-layer structure inwhich a titanium film or a titanium nitride film, an aluminum film or acopper film, and a titanium film or a titanium nitride film are stackedin this order; or a three-layer structure in which a molybdenum film ora molybdenum nitride film, an aluminum film or a copper film, and amolybdenum film or a molybdenum nitride film are stacked in this ordercan be employed. For example, in the case where the conductive layer hasa three-layer structure, it is preferable that each of the first andthird layers be a film formed of titanium, titanium nitride, molybdenum,tungsten, an alloy containing molybdenum and tungsten, an alloycontaining molybdenum and zirconium, or molybdenum nitride, and that thesecond layer be a film formed of a low-resistance material such ascopper, aluminum, gold, silver, or an alloy containing copper andmanganese. Note that light-transmitting conductive materials such asITO, indium oxide containing tungsten oxide, indium zinc oxidecontaining tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or ITSOmay be used.

An oxide conductive layer may be formed by controlling the resistivityof the oxide semiconductor.

<<Adhesive Layer 141>>

A curable resin such as a heat-curable resin, a photocurable resin, or atwo-component type curable resin can be used for the adhesive layer 141.For example, an acrylic resin, a urethane resin, an epoxy resin, or asiloxane resin can be used.

<<Connector 242>>

As the connector 242, for example, an anisotropic conductive film (ACF),an anisotropic conductive paste (ACP), and the like can be used.

<<Coloring Layer 131>>

The coloring layer 131 is a colored layer that transmits light in aspecific wavelength range. Examples of materials that can be used forthe coloring layer 131 include a metal material, a resin material, and aresin material containing a pigment or dye.

<<Light-Blocking Layer 132>>

The light-blocking layer 132 is provided, for example, between adjacentcoloring layers 131 for different colors. A black matrix formed with,for example, a metal material or a resin material containing a pigmentor dye can be used as the light-blocking layer 132. Note that it ispreferable to provide the light-blocking layer 132 also in a regionother than the display portion 62, such as the driver circuit portion64, in which case leakage of guided light or the like can be inhibited.

The thin films constituting the display device (i.e., the insulatingfilm, the semiconductor film, the conductive film, and the like) can beformed by any of a sputtering method, a chemical vapor deposition (CVD)method, a vacuum evaporation method, a pulsed laser deposition (PLD)method, an atomic layer deposition (ALD) method, and the like. Asexamples of a CVD method, a plasma-enhanced CVD (PECVD) method, athermal CVD method, or the like can be given. As an example of thethermal CVD method, metal organic CVD (MOCVD) method can be given.

Alternatively, the thin films constituting the display device (i.e., theinsulating film, the semiconductor film, the conductive film, and thelike) can be formed by a method such as spin coating, dipping, spraycoating, inkjet printing, dispensing, screen printing, or offsetprinting, or with a doctor knife, a slit coater, a roll coater, acurtain coater, or a knife coater.

The thin films constituting the display device can be processed using aphotolithography method or the like. Alternatively, island-shaped thinfilms may be formed by a film formation method using a blocking mask.Alternatively, the thin films may be processed by a nano-imprintingmethod, a sandblasting method, a lift-off method, or the like. As thephotolithography method, there are a method in which a resist mask isformed over a thin film to be processed, the thin film is processed byetching or the like, and the resist mask is removed and a method inwhich a photosensitive thin film is formed, and the photosensitive thinfilm is exposed to light and developed to be processed in a desirableshape.

As light used in exposure in a photolithography method, light with ani-line (with a wavelength of 365 nm), light with a g-line (with awavelength of 436 nm), light with an h-line (with a wavelength of 405nm), and light in which the i-line, the g-line, and the h-line are mixedcan be given. Alternatively, ultraviolet light, KrF laser light, ArFlaser light, or the like can be used. Exposure may be performed byliquid immersion exposure technique. As light used in exposure, extremeultra-violet light (EUV), X-rays or the like can be given. Instead ofthe light for the exposure, an electron beam can be used. It ispreferable to use extreme ultra-violet light, X-rays, or an electronbeam because extremely minute processing can be performed. Note that inthe case of performing exposure by scanning of a beam such as anelectron beam, a photomask is not needed.

For etching of the thin films, dry etching, wet etching, a sandblastmethod, or the like can be used.

<2. Structure Example 2 of Display Device>

FIG. 6, FIG. 7, and FIGS. 8A to 8D show examples of the display device.FIG. 6 is a cross-sectional view of a display device 100B. FIG. 7 is across-sectional view of a display device 100C. FIG. 8A is across-sectional view of a display device 100D. Note that the perspectiveviews of the display devices 100B, 100C, and 100D are not drawn here, asthey are similar to the perspective view of the display device 100A,which is shown in FIG. 1.

The display device 100B shown in FIG. 6 is different from the displaydevice 100A in the structure of the transistor.

Specifically, the transistors 201 and 206 in the display device 100Beach include two gates, though the transistor in the display device 100Aincludes only one gate. As described above, it is preferable that thetwo gates be electrically connected to each other. Thus, thefield-effect mobility of the transistor can be increased.

Other components are similar to those in the display device 100A; thus,the detailed description thereof is omitted.

The display device 100C shown in FIG. 7 is an example of a transmissiveliquid crystal display device that includes a liquid crystal elementwith a vertical electric field mode.

As shown in FIG. 7, the display device 100C includes the substrate 51,the transistor 201, the transistor 206, the liquid crystal element 40, acapacitor 219, the alignment film 133 a, the alignment film 133 b, theconnection portion 204, the adhesive layer 141, the coloring layer 131,the light-blocking layer 132, the overcoat 121, the substrate 61, thepolarizer 130, and the like.

The display portion 62 includes the transistor 206, the liquid crystalelement 40, and the capacitor 219.

The transistor 206 includes the gate 221, the insulating layer 213, theconductive layer 222 a, the conductive layer 222 c, and thesemiconductor layer 231.

Each of the conductive layers 222 a and 222 c is connected to thesemiconductor layer 231.

The liquid crystal element 40 is a liquid crystal element with a VAmode. The liquid crystal element 40 includes the pixel electrode 111,the common electrode 112, and the liquid crystal layer 113. The liquidcrystal layer 113 is positioned between the pixel electrode 111 and thecommon electrode 112.

The pixel electrode 111 is electrically connected to the semiconductorlayer 231 of the transistor 206 with the conductive layer 222 cpositioned therebetween.

The capacitor 219 includes a conductive layer 217 and a conductive layer218. The conductive layer 217 and the conductive layer 218 overlap witheach other with the insulating layer 213 positioned therebetween.

The semiconductor layer 231, the conductive layer 222 c, the conductivelayer 217, and the conductive layer 218 are formed using a conductivematerial that transmits visible light. The conductive layer 218 and theconductive layer 222 c can be formed using the same manufacturing stepand the same material. Hence, the contact portion where the pixelelectrode 111 and the transistor 206 are in contact with each other andthe capacitor 219 can be provided in the display region 68. Accordingly,the aperture ratio can be increased.

When the overcoat 121 has a planarization function, the common electrode112 can be provided flatly. This allows a thickness variation of theliquid crystal layer 113 to be reduced.

Examples of materials and formation methods of layers in the transistor206 shown in FIG. 7 are described.

First, a conductive film that transmits visible light is formed as oneelectrode of the capacitor (the conductive layer 217), and a metal filmsuch as a Cu film is formed by a sputtering method as the gate 221. Themetal film also functions as a scan line. With the use of this metalfilm, the gate 221 and a gate wiring of a transistor in the peripheralcircuit can be formed in the same manufacturing step.

Then, a stack of a silicon nitride film and a silicon oxynitride film isformed as the insulating layer 213 functioning as a gate insulatinglayer. Then, a stack of a CAC-OS film and a CAAC-OS film is formed by asputtering method as the semiconductor layer 231. In the case where theCAAC-OS film with high chemical solution resistance and high plasmaresistance is formed over the CAC-OS film, the semiconductor layer 231is less damaged in the manufacturing process of the transistor. Then, anindium zinc oxide film is formed by a sputtering method as theconductive layer 222 c functioning as a source electrode or a drainelectrode. The semiconductor layer 231 and the conductive layer 222 ccan be formed by wet etching. In order to increase the selectivity sothat the semiconductor layer 231 is not etched at the time of formingthe conductive layer 222 c, the conductive layer 222 c is preferablyformed using an etchant different from that used for forming thesemiconductor layer 231. With the use of this indium zinc oxide film,the conductive layer 222 c and the other electrode of the capacitor (theconductive layer 218) can be formed using the same manufacturing step.

Then, a metal film such as a Cu film is formed by a sputtering method asthe signal line and the conductive layer 222 a. With the use of thismetal film, the signal line, the conductive layer 222 a, and a sourcewiring and a drain wiring of a transistor in the peripheral circuit canbe formed using the same manufacturing step.

Then, the insulating layer 212 and the insulating layer 214 functioningas a passivation film are formed by stacking a silicon oxynitride filmand a silicon nitride film using a PECVD apparatus. Then, an acrylicresin is applied as the insulating layer 215 having a planarizationfunction, and an opening portion (a contact opening) is formed. Then, anITO film is formed as the pixel electrode 111.

Note that the metal film such as a Cu film, which is formed as the scanline, is preferably used for the gate electrode of the transistorincluded in the pixel. The metal film can suppress irradiation of achannel formation region with light emitted from the backlight. In FIG.7, the contact portion where the transistor 206 and the pixel electrode111 are in contact with each other and the capacitor 219 can transmitvisible light.

The display device 100D shown in FIG. 8A is different from theabove-described display device 100C in the positions and the shapes ofthe pixel electrode 111 and the common electrode 112.

Both of the pixel electrode 111 and the common electrode 112 may have atop-surface shape (also referred to as a planar shape) that has acomb-like shape or a top-surface shape that is provided with a slit.

In the display device 100D illustrated in FIG. 8A, the pixel electrode111 and the common electrode 112 are provided on the same plane.

Alternatively, the electrodes may have a shape in which an edge of aslit in one electrode is aligned with an edge of a slit in the otherelectrode. The cross-sectional view of this case is shown in FIG. 8B.

Alternatively, the pixel electrode 111 and the common electrode 112 mayhave a portion overlapping with each other, when seen from above. Thecross-sectional view of this case is shown in FIG. 8C.

Alternatively, the display portion 62 may have a portion where neitherthe pixel electrode 111 nor the common electrode 112 is provided, whenseen from above. The cross-sectional view of this case is shown in FIG.8D.

As described above, the display device of one embodiment of the presentinvention can include transistors and liquid crystal elements withvarious shapes.

<3. Pixel Arrangement Example>

The pixel arrangement examples are shown in FIGS. 9A and 9B. FIGS. 9Aand 9B show examples in which one pixel is composed of a red subpixel R,a green subpixel G, and a blue subpixel B. In FIGS. 9A and 9B, aplurality of scan lines 81 extend in the x direction, and a plurality ofsignal lines 82 extend in the y direction. The scan lines 81 and thesignal lines 82 intersect with each other.

As shown by the dashed-two-dotted line in FIG. 9A, a subpixel includesthe transistor 206, a capacitor 34, and the liquid crystal element 40. Agate of the transistor 206 is electrically connected to the scan line81. One of a source and a drain of the transistor 206 is electricallyconnected to the signal line 82, and the other is electrically connectedto one electrode of the capacitor 34 and one electrode of the liquidcrystal element 40. The other electrode of the capacitor 34 and theother electrode of the liquid crystal element 40 are each supplied witha constant potential.

FIGS. 9A and 9B show examples where source-line inversion driving isadopted. Signals A1 and A2 are signals with the same polarity. SignalsB1 and B2 are signals with the same polarity. Signals A1 and B1 aresignals with different polarities. Signals A2 and B2 are signals withdifferent polarities.

As the definition of the display device becomes higher, the distancebetween subpixels becomes shorter. Thus, as shown in the frame outlinedin a dashed-dotted line in FIG. 9A, in the subpixel where the signal A1is input, the liquid crystal is easily affected by potentials in boththe signal A1 and the signal B1, in the vicinities of the signal line 82where the signal B1 is input. This can make the liquid crystal moreprone to alignment defects.

In FIG. 9A, the direction in which a plurality of subpixels exhibitingthe same color are aligned is the y direction, and is substantiallyparallel to the direction where the signal lines 82 extend. As shown inthe frame outlined in the dashed-dotted line in FIG. 9A, subpixelsexhibiting different colors are adjacent to each other, with the longersides of the subpixels facing each other.

In FIG. 9B, the direction in which a plurality of subpixels exhibitingthe same color are aligned is the x direction, and intersects with thedirection where the signal lines 82 extend. As shown in the frameoutlined in a dashed-dotted line in FIG. 9B, subpixels exhibiting thesame color are adjacent to each other, with the shorter sides of thesubpixels facing each other.

When the side of the subpixel that is substantially parallel to thedirection in which the signal lines 82 extend is the shorter side of thesubpixel as illustrated in FIG. 9B, the region where the liquid crystalis more prone to alignment defects can be made narrower, compared withthe case (illustrated in FIG. 9A) where the longer side of the subpixelis substantially parallel to the direction in which the signal lines 82extend. When the region where the liquid crystal is more prone toalignment defects is positioned between subpixels exhibiting the samecolor as illustrated in FIG. 9B, display defects are less easilyrecognized by a user of the display device when compared with the case(see FIG. 9A) where the region is positioned between subpixelsexhibiting different colors. In one embodiment of the present invention,the direction in which the plurality of subpixels exhibiting the samecolor are arranged preferably intersects with the direction in which thesignal lines 82 extend.

<4. Structure Example 3 of Display Device>

One embodiment of the present invention can be applied to a displaydevice in which a touch sensor is implemented; such a display device isalso referred to as an input/output device or a touch panel. Any of thestructures of the display device described above can be applied to thetouch panel. In this embodiment, the description focuses on an examplein which the touch sensor is implemented in the display device 100A.

There is no limitation on the sensing element (also referred to as asensor element) included in the touch panel of one embodiment of thepresent invention. A variety of sensors capable of sensing an approachor a contact of an object such as a finger or a stylus can be used asthe sensor element.

For example, a variety of types such as a capacitive type, a resistivetype, a surface acoustic wave type, an infrared type, an optical type,and a pressure-sensitive type can be used for the sensor.

In this embodiment, a touch panel including a capacitive sensor elementis described as an example.

Examples of the capacitive touch sensor element include a surfacecapacitive touch sensor element and a projected capacitive touch sensorelement. Examples of the projected capacitive sensor element include aself-capacitive sensor element and a mutual capacitive sensor element.The use of a mutual capacitive sensor element is preferable becausemultiple points can be sensed simultaneously.

The touch panel of one embodiment of the present invention can have anyof a variety of structures, including a structure in which a displaydevice and a sensor element that are separately formed are attached toeach other and a structure in which an electrode and the like includedin a sensor element are provided on one or both of a substratesupporting a display element and a counter substrate.

FIGS. 10A and 10B illustrate an example of the touch panel. FIG. 10A isa perspective view of a touch panel 350A. FIG. 10B is a developed viewof the schematic perspective view of FIG. 10A. Note that for simplicity,FIGS. 10A and 10B illustrate only the major components. In FIG. 10B, theoutlines of the substrate 61 and a substrate 162 are illustrated only indashed lines.

The touch panel 350A has a structure in which a display device and asensor element that are fabricated separately are bonded together.

The touch panel 350A includes an input device 375 and a display device370 that are provided to overlap with each other.

The input device 375 includes the substrate 162, an electrode 127, anelectrode 128, a plurality of wirings 137, and a plurality of wirings138. An FPC 72 b is electrically connected to each of the plurality ofwirings 137 and the plurality of wirings 138. An IC 73 b is provided onthe FPC 72 b.

The display device 370 includes the substrate 51 and the substrate 61which are provided to face each other. The display device 370 includesthe display portion 62 and the driver circuit portion 64. The wiring 65and the like are provided over the substrate 51. An FPC 72 a iselectrically connected to the wiring 65. An IC 73 a is provided on theFPC 72 a.

The wiring 65 supplies signals and power to the display portion 62 andthe driver circuit portion 64. The signals and power are input to thewiring 65 from the outside or the IC 73 a, through the FPC 72 a.

The display device 100A shown in FIG. 2A can be used as the displaydevice 370 shown in FIGS. 10A and 10B.

<5. Structure Example 4 of Display Device>

FIGS. 11A and 11B illustrate an example of the touch panel. FIG. 11A isa perspective view of a touch panel 350B. FIG. 11B is a developed viewof the schematic perspective view of FIG. 11A. Note that for simplicity,FIGS. 11A and 11B illustrate only the major components. In FIG. 11B, theoutline of the substrate 61 is illustrated only in a dashed line.

The touch panel 350B is an in-cell touch panel that has a function ofdisplaying an image and serves as a touch sensor.

The touch panel 350B has a structure in which electrodes constituting asensor element and the like are provided only on the counter substrate.Such a structure can make the touch panel thinner and more lightweightor reduce the number of components within the touch panel, compared witha structure in which the display device and the sensor element arefabricated separately and then are bonded together.

In FIGS. 11A and 11B, an input device 376 is provided on the substrate61. The wirings 137 and 138 and the like of the input device 376 areelectrically connected to the FPC 72 included in a display device 379.For example, in a connection portion 63, one of the wirings 137 (or thewirings 138) and the conductive layer provided on the substrate 51 sideare electrically connected to each other through a connector.

With the above structure, the FPCs connected to the touch panel 350B canbe provided only on one substrate side (on the substrate 51 side in thisembodiment). Although two or more FPCs may be attached to the touchpanel 350B, it is preferable that the touch panel 350B be provided withone FPC 72 which has a function of supplying signals to both the displaydevice 379 and the input device 376 as illustrated in FIGS. 11A and 11B,for the simplicity of the structure. The touch panel 350B can easily beincorporated into an electronic device and allows a reduction in thenumber of components compared with the case where FPCs are connected toboth the substrate 51 and the substrate 61.

The IC 73 may include a function of driving the input device 376.Another IC that drives the input device 376 may be provided over the FPC72. Alternatively, an IC that drives the input device 376 may be mountedon the substrate 51.

In the conductive layers included in the input device, the conductivelayers that overlap with the display region 68 are formed using amaterial that transmits visible light. Note that the conductive layersin the input device may be arranged only in the non-display region 66.When the conductive layer in the input device does not overlap with thedisplay region 68, the conductive layer does not need to be formed witha material that transmits visible light. A material with a lowresistivity such as a metal can be used for the conductive layerincluded in the input device. For example, the wiring and the electrodeof the touch sensor are preferably formed with a metal mesh, therebyhaving a lower resistance. In that case, the touch sensor is suitablyused in a large-sized display device. Note that a metal, which isgenerally a material having a high reflectivity, can be darkened bybeing subjected to oxidation treatment or the like. Thus, even when thedisplay device is seen from the display surface side, a decrease invisibility due to the reflection of external light can be suppressed.

The wiring and the electrode can be formed with a stack of a metal layerand a layer with a low reflectivity (the layer is also referred to as adark-colored layer). Examples of the dark-colored layer include a layercontaining copper oxide, and a layer containing copper chloride ortellurium chloride. Alternatively, the dark-colored layer may be formedwith a metal particle such as an Ag particle, an Ag fiber, or a Cuparticle, a carbon nanoparticle such as a carbon nanotube (CNT) orgraphene, a conductive high molecule such as PEDOT, polyaniline, orpolypyrrole, or the like.

The display device in this embodiment includes the region where thetransistor transmits visible light, allowing an increase in the apertureratio of the pixel. Thus, the power consumption of the display devicecan be reduced.

This embodiment can be combined with any of other embodiments asappropriate. In the case where a plurality of structure examples aredescribed in one embodiment in this specification, some of the structureexamples can be combined as appropriate.

Embodiment 2

In this embodiment, an operation mode which can be employed in thedisplay device of one embodiment of the present invention is describedwith reference to FIGS. 12A to 12C.

A normal driving mode (Normal mode) with a normal frame frequency(typically, higher than or equal to 60 Hz and lower than or equal to 240Hz) and an idling stop (IDS) driving mode with a low frame frequency aredescribed below.

Note that the IDS driving mode refers to a driving method in which afterimage data is written, rewriting of image data is stopped. Thisincreases the interval between writing of image data and subsequentwriting of image data, thereby reducing the power that would be consumedby writing of image data in that interval. The IDS driving mode can beperformed at a frame frequency which is 1/100 to 1/10 of the normaldriving mode, for example. A still image is displayed by the same videosignals in consecutive frames. Thus, the IDS driving mode isparticularly effective when displaying a still image. When an image isdisplayed using IDS driving, power consumption is reduced, imageflickering (flicker) is suppressed, and eyestrain can be reduced.

FIG. 12A is a pixel circuit diagram, FIGS. 12B and 12C are timing chartsshowing a normal driving mode and an IDS driving mode. Note that in FIG.12A, a first display element 501 (here, a reflective liquid crystalelement) and a pixel circuit 506 electrically connected to the firstdisplay element 501 are illustrated. In the pixel circuit 506illustrated in FIG. 12A, a signal line SL, a gate line GL, a transistorM1 connected to the signal line SL and the gate line GL, and a capacitorCs_(LC) connected to the transistor M1 are illustrated.

The transistor M1 may become a leakage path of data D₁. Accordingly, theoff-state current of the transistor M1 is preferably as low as possible.A transistor including a metal oxide in a semiconductor layer in which achannel is formed is preferably used as the transistor M1. A metal oxidehaving at least one of an amplification function, a rectificationfunction, and a switching function can be referred to as a metal oxidesemiconductor or an oxide semiconductor (abbreviated to an OS). As atypical example of a transistor, a transistor including an oxidesemiconductor in a semiconductor layer in which a channel is formed (OStransistor) is described. The OS transistor has a feature of extremelylow leakage current (off-state current) in an off state compared with atransistor including polycrystalline silicon or the like. When the OStransistor is used as the transistor M1, electric charges supplied to anode ND1 can be held for a long period.

In the circuit diagram illustrated in FIG. 12A, the liquid crystalelement LC becomes a leakage path of data D₁. Therefore, to perform IDSdriving appropriately, the resistivity of the liquid crystal element LCis preferably higher than or equal to 1.0×10¹⁴ Ω·cm.

Note that for example, an oxide including In, Ga, and Zn, an oxideincluding In and Zn, or the like can be suitably used for a channelregion of the above OS transistor. The oxide including In, Ga, and Zncan typically have an atomic ratio of In:Ga:Zn=4:2:4.1 or a neighborhoodthereof.

FIG. 12B is a timing chart showing the waveforms of signals supplied tothe signal line SL and the gate line GL in the normal driving mode. Inthe normal driving mode, a normal frame frequency (e.g., 60 Hz) is usedfor operation. FIG. 12B shows a period T₁, a period T₂, and a period T₃.A scan signal is supplied to the gate line GL in each frame period anddata D₁ is written from the signal line SL to the node ND1. Thisoperation is performed both to write the same data D₁ in the periods T₁to T₃ and to write different data in the periods T₁ to T₃.

FIG. 12C is a timing chart showing the waveforms of signals supplied tothe signal line SL and the gate line GL in the IDS driving mode. In theIDS driving, a low frame frequency (e.g., 1 Hz) is used for operation.One frame period is denoted by a period T₁ and includes a data writingperiod T_(W) and a data retention period T_(RET). In the IDS drivingmode, a scan signal is supplied to the gate line GL and the data D₁ ofthe signal line SL is written in the period T_(W), the gate line GL isfixed to a low-level voltage in the period T_(RET), and the transistorM1 is turned off so that the written data D₁ is retained. Note that thelow frame frequency may be higher than or equal to 0.1 Hz and lower than60 Hz, for example.

This embodiment can be combined with any of other embodiments asappropriate.

Embodiment 3

In this embodiment, a driving method of a touch sensor is described withreference to drawings.

<Example of Sensing Method of Sensor>

FIG. 13A is a block diagram illustrating the structure of a mutualcapacitive touch sensor. FIG. 13A illustrates a pulse voltage outputcircuit 551 and a current sensing circuit 552. Note that in FIG. 13A,six wirings X1 to X6 represent electrodes 521 to which a pulse voltageis applied, and six wirings Y1 to Y6 represent electrodes 522 that sensechanges in current. FIG. 13A also illustrates a capacitor 553 that isformed where the electrodes 521 and 522 overlap with each other. Notethat functional replacement between the electrodes 521 and 522 ispossible.

The pulse voltage output circuit 551 is a circuit for sequentiallyapplying a pulse voltage to the wirings X1 to X6. By application of apulse voltage to the wirings X1 to X6, an electric field is generatedbetween the electrodes 521 and 522 of the capacitors 553. When theelectric field between the electrodes is shielded, for example, a changeoccurs in the capacitor 553 (mutual capacitance). The approach orcontact of a sensing target can be sensed by utilizing this change.

The current sensing circuit 552 is a circuit for detecting changes incurrent flowing through the wirings Y1 to Y6 that are caused by thechange in mutual capacitance in the capacitor 553. No change in currentvalue is detected in the wirings Y1 to Y6 when there is no approach orcontact of a sensing target, whereas a decrease in current value isdetected when mutual capacitance is decreased owing to the approach orcontact of a sensing target. Note that an integrator circuit or the likeis used for sensing of current.

Note that one or both of the pulse voltage output circuit 551 and thecurrent sensing circuit 552 may be formed over the substrate 51 or thesubstrate 61 that is shown in FIG. 1 or the like. For example, it ispreferable to form the display portion 62, the driver circuit portion64, and the like at the same time because the process can be simplifiedand the number of components used for driving the touch sensor can bereduced. One or both of the pulse voltage output circuit 551 and thecurrent sensing circuit 552 may be mounted on the IC 73.

In particular, in the case of using crystalline silicon such aspolycrystalline silicon or single crystal silicon for the semiconductorlayer where a channel is formed in the transistor over the substrate 51,driving characteristics of the pulse voltage output circuit 551, thecurrent sensing circuit 552, or the like are increased and sensitivityof the touch sensor can be thus increased.

FIG. 13B is a timing chart showing input and output waveforms in themutual capacitive touch sensor illustrated in FIG. 13A. In FIG. 13B,sensing of a sensing target is performed in all the rows and columns inone frame period. FIG. 13B shows a period when a sensing target is notsensed (not touched) and a period when a sensing target is sensed(touched). Sensed current values of the wirings Y1 to Y6 are shown asthe waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and thewaveforms of the wirings Y1 to Y6 change in accordance with the pulsevoltage. When there is no approach or contact of a sensing target, thewaveforms of the wirings Y1 to Y6 change uniformly in accordance withchanges in the voltages of the wirings X1 to X6. The current value isdecreased at the point of approach or contact of a sensing target andaccordingly the waveform of the voltage value changes.

By sensing a change in mutual capacitance in this manner, the approachor contact of a sensing target can be sensed.

<Example of Driving Method of Display Device>

FIG. 14A is a block diagram illustrating a configuration example of adisplay device. FIG. 14A illustrates a gate driver circuit GD (a scanline driver circuit), a source driver circuit SD (a signal line drivercircuit), and a display portion including a plurality of pixels pix. InFIG. 14A, gate lines x_1 to x_m (m is a natural number) electricallyconnected to the gate driver circuit GD and source lines y_1 to y_n (nis a natural number) electrically connected to the source driver circuitSD are shown. Corresponding to these lines, the pixels pix are denotedby (1, 1) to (n, m).

FIG. 14B is a timing chart of signals supplied to the gate lines and thesource lines in the display device shown in FIG. 14A. The periods inFIG. 14B show the case where data signals are rewritten every frameperiod and the case where data signals are not rewritten. Note thatperiods such as a retrace period are not taken into consideration inFIG. 14B.

In the case where data signals are rewritten every frame period, scansignals are sequentially supplied to the gate lines x_1 to x_m. In ahorizontal scanning period 1H, during which the scan signal is at an Hlevel, data signals D are supplied to the source lines y_1 to y_n in thecolumns.

In the case where data signals are not rewritten every frame period,supply of scan signals to the gate lines x_1 to x_m is stopped. In thehorizontal scanning period 1H, supply of data signals to the sourcelines y_1 to y_n in the columns is stopped.

A driving method in which data signals are not rewritten every frameperiod is effective particularly when an oxide semiconductor is used forthe semiconductor layer where a channel is formed in the transistorincluded in the pixel pix. A transistor including an oxide semiconductorcan have much lower off-state current than a transistor including asemiconductor such as silicon. Thus, a data signal written in theprevious period can be held without rewriting data signals every frameperiod, and grayscale of pixels can be held for 1 second or longer,preferably 5 seconds or longer, for example.

In the case where polycrystalline silicon or the like is used for asemiconductor layer where a channel of a transistor included in thepixel pix is formed, the storage capacitance of the pixel is preferablyincreased in advance. The larger the storage capacitance is, the longerthe grayscale of the pixel can be held. The storage capacitance may bedetermined depending on leakage current of a transistor or a displayelement which is electrically connected to the storage capacitor. Forexample, the storage capacitance per pixel is set to 5 fF to 5 pFinclusive, preferably 10 fF to 5 pF inclusive, further preferably 20 fFto 1 pF inclusive, so that a data signal written in the previous periodcan be held without rewriting data signals every frame period. Forexample, grayscale of a pixel can be held for several frame periods orseveral tens of frame periods.

<Example of Driving Method of Display Portion and Touch Sensor>

FIGS. 15A to 15D show examples of the operations in successive frameperiods of the touch sensor described with reference to FIGS. 13A and13B and the display portion described with reference to FIGS. 14A and14B that are driven for 1 sec (one second). In FIG. 15A, one frameperiod for the display portion is 16.7 ms (frame frequency: 60 Hz), andone frame period for the touch sensor is 16.7 ms (frame frequency: 60Hz).

In the display device of one embodiment of the present invention, thedisplay portion and the touch sensor operate independently of eachother, and the display device can have a touch sensing period concurrentwith a display period. That is why one frame period for the displayportion and one frame period for the touch sensor can both be 16.7 ms(frame frequency: 60 Hz) as shown in FIG. 15A. The frame frequency forthe touch sensor may differ from that of the display portion. Forexample, as shown in FIG. 15B, one frame period for the display portionmay be 8.3 ms (frame frequency: 120 Hz) and one frame period for thetouch sensor may be 16.7 ms (frame frequency: 60 Hz). The framefrequency for the display portion may be 33.3 ms (frame frequency: 30Hz) (not shown).

The frame frequency for the display portion may be changeable, i.e., theframe frequency in displaying moving images may be increased (e.g., 60Hz or more, or 120 Hz or more), whereas the frame frequency indisplaying still images may be decreased (e.g., 60 Hz or less, 30 Hz orless, or 1 Hz or less). With this structure, power consumption of thedisplay device can be reduced. The frame frequency for the touch sensormay be changeable, and the frame frequency in waiting may differ fromthe frame frequency in sensing a touch.

Moreover, in the display device of one embodiment of the presentinvention, the following operation is possible: data signals are notrewritten in the display portion and a data signal written in theprevious period is held. In that case, one frame period of the displayportion can be longer than 16.7 ms. Thus, as shown in FIG. 15C, theoperation can be switched so that one frame period for the displayportion is 1 sec (frame frequency: 1 Hz) and one frame period for thetouch sensor is 16.7 ms (frame frequency: 60 Hz).

Note that for the operation in which data signals are not rewritten inthe display portion and a data signal written in the previous period isheld, the above-described IDS driving mode can be referred to. As theIDS driving mode, a partial IDS driving mode may be employed in whichdata signals are rewritten only in a specific region of the displayportion. The partial IDS driving mode is a mode in which data signalsare rewritten only in a specific region of the display portion and adata signal written in the previous period is held in the other region.

Furthermore, by the driving method of a touch sensor that is disclosedin this embodiment, the touch sensor can be continuously driven in thecase of FIG. 15C. Thus, data signals in the display portion can also berewritten when the approach or contact of a sensing target is sensed bythe touch sensor, as shown in FIG. 15D.

If rewriting of data signals in a display portion is performed during asensing period of a touch sensor, noise caused by rewriting of the datasignals travels through the touch sensor and the sensitivity of thetouch sensor might decrease. For this reason, rewriting of data signalsin a display portion and sensing by a touch sensor are preferablyperformed in different periods.

FIG. 16A shows an example in which rewriting of data signals in adisplay portion and sensing by a touch sensor are performed alternately.FIG. 16B shows an example in which sensing by a touch sensor isperformed one time every two rewritings of data signals in a displayportion. Note that sensing by a touch sensor may be performed once everythree or more rewritings.

In the case where an oxide semiconductor is used in a semiconductorlayer (where a channel is formed) of a transistor used in the pixel pix,off-state current can be significantly reduced and the frequency ofrewriting data signal can be sufficiently reduced. Specifically, asufficiently long break period can be set between rewritings of datasignals. The break period can be 0.5 seconds or longer, 1 second orlonger, or 5 seconds or longer, for example. The upper limit of thebreak period depends on leakage current of a capacitor or a displayelement connected to a transistor; for example, 1 minute or shorter, 10minutes or shorter, 1 hour or shorter, or 1 day or shorter.

FIG. 16C shows an example in which rewriting of data signals in adisplay portion is performed once every 5 seconds. A break period forstopping the rewriting operation of a display portion is set in FIG. 16Cbetween rewriting of data signals and next rewriting. In the breakperiod, a touch sensor can be operated at a frame frequency of i Hz (iis more than or equal to the frame frequency of a display device; here,0.2 Hz or more). Sensing by a touch sensor is performed in a breakperiod and is not performed in a rewriting period of data signals in adisplay portion as shown in FIG. 16C, so that sensitivity of a touchsensor can be increased. When rewriting of data signals in a displayportion and sensing by a touch sensor are performed at the same time asshown in FIG. 16D, operation signals can be simplified.

In a break period during which rewriting of data signals in a displayportion is not performed, not only the supply of data signals to thedisplay portion, but also the operation of one or both of the gatedriver circuit GD and the source driver circuit SD may be stopped. Thesupply of power to one or both of the gate driver circuit GD and thesource driver circuit SD may also be stopped. Thus, noise is furtherreduced, and the sensitivity of the touch sensor can be furtherincreased. Moreover, the power consumption of the display device can befurther reduced.

The display device of one embodiment of the present invention includes adisplay portion and a touch sensor between two substrates. With thisstructure, the distance between the display portion and the touch sensorcan be reduced. At this time, noise is easily transmitted to the touchsensor in driving the display portion, which might reduce thesensitivity of the touch sensor. When the driving method in thisembodiment is employed, a display device including a touch sensor, whichhas both reduced thickness and high sensitivity, can be obtained.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 4

Described in this embodiment is a metal oxide that can be used in asemiconductor layer of a transistor disclosed in one embodiment of thepresent invention. Note that in the case where a metal oxide is used ina semiconductor layer of a transistor, the metal oxide can be rephrasedas an oxide semiconductor.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofthe non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

A cloud-aligned composite OS (CAC-OS) may be used in a semiconductorlayer of a transistor disclosed in one embodiment of the presentinvention.

The aforementioned non-single-crystal oxide semiconductor or CAC-OS canbe suitably used in a semiconductor layer of a transistor disclosed inone embodiment of the present invention. As the non-single-crystal oxidesemiconductor, an nc-OS or a CAAC-OS can be suitably used.

In one embodiment of the present invention, a CAC-OS is preferably usedin a semiconductor layer of a transistor. The use of the CAC-OS allowsthe transistor to have high electrical characteristics or highreliability.

The CAC-OS will be described in detail below.

A CAC-OS or a CAC metal oxide has a conducting function in a part of thematerial and has an insulating function in another part of the material;as a whole, the CAC-OS or the CAC metal oxide has a function of asemiconductor. In the case where the CAC-OS or the CAC metal oxide isused in a channel formation region of a transistor, the conductingfunction is to allow electrons (or holes) serving as carriers to flow,and the insulating function is to not allow electrons serving ascarriers to flow. By the complementary action of the conducting functionand the insulating function, the CAC-OS or the CAC metal oxide can havea switching function (on/off function). In the CAC-OS or the CAC metaloxide, separation of the functions can maximize each function.

The CAC-OS or the CAC metal oxide includes conductive regions andinsulating regions. The conductive regions have the aforementionedconducting function and the insulating regions have the aforementionedinsulating function. In some cases, the conductive regions and theinsulating regions in the material are separated at the nanoparticlelevel. In some cases, the conductive regions and the insulating regionsare unevenly distributed in the material. The conductive regions aresometimes observed to be coupled in a cloud-like manner with theirboundaries blurred.

In the CAC-OS or the CAC metal oxide, the conductive regions and theinsulating regions each have a size greater than or equal to 0.5 nm andless than or equal to 10 nm, preferably greater than or equal to 0.5 nmand less than or equal to 3 nm and are dispersed in the material, insome cases.

The CAC-OS or the CAC metal oxide includes components having differentbandgaps. For example, the CAC-OS or the CAC metal oxide includes acomponent having a wide gap due to the insulating region and a componenthaving a narrow gap due to the conductive region. In the case of such acomposition, carriers mainly flow in the component having a narrow gap.The component having a narrow gap complements the component having awide gap, and carriers also flow in the component having a wide gap inconjunction with the component having a narrow gap. Therefore, in thecase where the above-described CAC-OS or CAC metal oxide is used in achannel formation region of a transistor, high current drive capabilityin the on state of the transistor, that is, high on-state current andhigh field-effect mobility, can be obtained.

In other words, the CAC-OS or the CAC-metal oxide can be referred to asa matrix composite or a metal matrix composite.

The CAC-OS has, for example, a composition in which elements included ina metal oxide are unevenly distributed. The unevenly distributedelements each have a size greater than or equal to 0.5 nm and less thanor equal to 10 nm, preferably greater than or equal to 1 nm and lessthan or equal to 2 nm, or a similar size. Note that in the followingdescription of a metal oxide, a state in which one or more metalelements are unevenly distributed and regions including the metalelement(s) are mixed is referred to as a mosaic pattern or a patch-likepattern. The regions each have a size greater than or equal to 0.5 nmand less than or equal to 10 nm, preferably greater than or equal to 1nm and less than or equal to 2 nm, or a similar size.

Note that a metal oxide preferably contains at least indium. Inparticular, indium and zinc are preferably contained. In addition, oneor more elements selected from aluminum, gallium, yttrium, copper,vanadium, beryllium, boron, silicon, titanium, iron, nickel, germanium,zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum,tungsten, magnesium, and the like may be contained.

For example, of the CAC-OS, an In—Ga—Zn oxide with the CAC composition(such an In—Ga—Zn oxide may be particularly referred to as CAC-IGZO) hasa composition in which materials are separated into indium oxide(InO_(X1), where X1 is a real number greater than 0) or indium zincoxide (In_(X2)Zn_(Y2)O_(Z2), where X2, Y2, and Z2 are real numbersgreater than 0), and gallium oxide (GaO_(X3), where X3 is a real numbergreater than 0) or gallium zinc oxide (Ga_(X4)Zn_(Y4)O_(Z4), where X4,Y4, and Z4 are real numbers greater than 0), and a mosaic pattern isformed. Then, InO_(X1) or In_(X2)Zn_(Y2)O_(Z2) forming the mosaicpattern is evenly distributed in the film. This composition is alsoreferred to as a cloud-like composition.

That is, the CAC-OS is a composite metal oxide with a composition inwhich a region including GaO_(X3) as a main component and a regionincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component aremixed. Note that in this specification, for example, when the atomicratio of In to an element M in a first region is greater than the atomicratio of In to an element M in a second region, the first region hashigher In concentration than the second region.

Note that a compound including In, Ga, Zn, and O is also known as IGZO.Typical examples of IGZO include a crystalline compound represented byInGaO₃(ZnO)_(m1) (m1 is a natural number) and a crystalline compoundrepresented by In_((1+x0))Ga_((1−x0))O₃(ZnO)_(m0) (−1≤x0≤1; m0 is agiven number).

The above crystalline compounds have a single crystal structure, apolycrystalline structure, or a c-axis-aligned crystalline (CAAC)structure. Note that the CAAC structure is a crystal structure in whicha plurality of IGZO nanocrystals have c-axis alignment and are connectedin the a-b plane direction without alignment.

On the other hand, the CAC-OS relates to the material composition of ametal oxide. In a material composition of a CAC-OS including In, Ga, Zn,and O, nanoparticle regions including Ga as a main component areobserved in part of the CAC-OS and nanoparticle regions including In asa main component are observed in part thereof. These nanoparticleregions are randomly dispersed to form a mosaic pattern. Therefore, thecrystal structure is a secondary element for the CAC-OS.

Note that in the CAC-OS, a stacked-layer structure including two or morefilms with different atomic ratios is not included. For example, atwo-layer structure of a film including In as a main component and afilm including Ga as a main component is not included.

A boundary between the region including GaO_(X3) as a main component andthe region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent is not clearly observed in some cases.

In the case where one or more of aluminum, yttrium, copper, vanadium,beryllium, boron, silicon, titanium, iron, nickel, germanium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,magnesium, and the like are contained instead of gallium in a CAC-OS,nanoparticle regions including the selected metal element(s) as a maincomponent(s) are observed in part of the CAC-OS and nanoparticle regionsincluding In as a main component are observed in part thereof, and thesenanoparticle regions are randomly dispersed to form a mosaic pattern inthe CAC-OS.

The CAC-OS can be formed by a sputtering method under conditions where asubstrate is not heated intentionally, for example. In the case offorming the CAC-OS by a sputtering method, one or more selected from aninert gas (typically, argon), an oxygen gas, and a nitrogen gas may beused as a deposition gas. The ratio of the flow rate of an oxygen gas tothe total flow rate of the deposition gas at the time of deposition ispreferably as low as possible, and for example, the flow ratio of anoxygen gas is preferably higher than or equal to 0% and less than 30%,further preferably higher than or equal to 0% and less than or equal to10%.

The CAC-OS is characterized in that no clear peak is observed inmeasurement using θ/2θ scan by an out-of-plane method, which is an X-raydiffraction (XRD) measurement method. That is, X-ray diffraction showsno alignment in the a-b plane direction and the c-axis direction in ameasured region.

In an electron diffraction pattern of the CAC-OS which is obtained byirradiation with an electron beam with a probe diameter of 1 nm (alsoreferred to as a nanometer-sized electron beam), a ring-like region withhigh luminance and a plurality of bright spots in the ring-like regionare observed. Therefore, the electron diffraction pattern indicates thatthe crystal structure of the CAC-OS includes a nanocrystal (nc)structure with no alignment in plan-view and cross-sectional directions.

For example, an energy dispersive X-ray spectroscopy (EDX) mapping imageconfirms that an In—Ga—Zn oxide with the CAC composition has a structurein which a region including GaO_(X3) as a main component and a regionincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component areunevenly distributed and mixed.

The CAC-OS has a structure different from that of an IGZO compound inwhich metal elements are evenly distributed, and has characteristicsdifferent from those of the IGZO compound. That is, in the CAC-OS,regions including GaO_(X3) or the like as a main component and regionsincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component areseparated to form a mosaic pattern.

The conductivity of a region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1)as a main component is higher than that of a region including GaO_(X3)or the like as a main component. In other words, when carriers flowthrough regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent, the conductivity of an oxide semiconductor is exhibited.Accordingly, when regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) asa main component are distributed in an oxide semiconductor like a cloud,high field-effect mobility (μ) can be achieved.

In contrast, the insulating property of a region including GaO_(X3) orthe like as a main component is higher than that of a region includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component. In other words,when regions including GaO_(X3) or the like as a main component aredistributed in an oxide semiconductor, leakage current can be suppressedand favorable switching operation can be achieved.

Accordingly, when a CAC-OS is used for a semiconductor element, theinsulating property derived from GaO_(X3) or the like and theconductivity derived from In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) complementeach other, whereby high on-state current (Ion) and high field-effectmobility (μ) can be achieved.

A semiconductor element including a CAC-OS has high reliability. Thus,the CAC-OS is suitably used in a variety of semiconductor devicestypified by a display.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 5

In this embodiment, electronic devices of one embodiment of the presentinvention are described.

Examples of electronic devices include a television set, a desktop orlaptop personal computer, a monitor of a computer or the like, a digitalcamera, a digital video camera, a digital photo frame, a mobile phone, aportable game machine, a portable information terminal, an audioreproducing device, and a large game machine such as a pachinko machine.

FIGS. 17A to 17C illustrate portable information terminals. Each of theportable information terminals in this embodiment functions as, forexample, one or more of a telephone set, a notebook, and an informationbrowsing system. Specifically, each of the portable informationterminals in this embodiment can be used as a smartphone or a smartwatch. Each of the portable information terminals in this embodiment iscapable of executing a variety of applications such as mobile phonecalls, e-mailing, text reading and editing, music replay, video replay,Internet communication, and a game, for example. Each of the portableinformation terminals illustrated in FIGS. 17A to 17C can have a varietyof functions. The portable information terminals illustrated in FIGS.17A to 17C can have a variety of functions, for example, a function ofdisplaying a variety of data (a still image, a moving image, a textimage, and the like) on the display portion, a touch panel function, afunction of displaying a calendar, date, time, and the like, a functionof controlling a process with a variety of software (programs), awireless communication function, a function of being connected to avariety of computer networks with a wireless communication function, afunction of transmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Note that the functions of the portableinformation terminals illustrated in FIGS. 17A to 17C are not limited tothe above, and the portable information terminals may have otherfunctions.

Each of the portable information terminals illustrated in FIGS. 17A to17C is capable of executing a variety of applications such as mobilephone calls, e-mailing, text reading and editing, music replay, Internetcommunication, and a computer game, for example. Each of the portableinformation terminals illustrated in FIGS. 17A to 17C can employ nearfield communication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 820 illustrated in FIG. 17Cand a headset capable of wireless communication can be performed, andthus hands-free calling is possible.

A portable information terminal 800 illustrated in FIG. 17A includes ahousing 811, a display portion 812, operation buttons 813, an externalconnection port 814, a speaker 815, a microphone 816, and the like. Thedisplay portion 812 of the portable information terminal 800 has a flatsurface.

A portable information terminal 810 illustrated in FIG. 17B includes thehousing 811, the display portion 812, the operation buttons 813, theexternal connection port 814, the speaker 815, the microphone 816, acamera 817, and the like. The display portion 812 of the portableinformation terminal 810 has a curved surface.

FIG. 17C illustrates a wrist-watch-type portable information terminal820. The portable information terminal 820 includes the housing 811, thedisplay portion 812, the speaker 815, operation keys 818 (including apower switch or an operation switch), and the like. The external shapeof the display portion 812 of the portable information terminal 820 iscircular. The display portion 812 of the portable information terminal820 has a flat surface.

The display device of one embodiment of the present invention can beused for the display portion 812. Thus, the display portion of theportable information terminal can have a high aperture ratio.

Each of the portable information terminals in this embodiment includes atouch sensor in the display portion 812. Operations such as making acall and inputting a letter can be performed by touch on the displayportion 812 with a finger, a stylus, or the like.

With the operation button 813, the power can be turned on or off. Inaddition, types of images displayed on the display portion 812 can beswitched; for example, switching images from a mail creation screen to amain menu screen is performed with the operation button 813.

When a detection device such as a gyroscope sensor or an accelerationsensor is provided inside each of the portable information terminals,the direction of display on the screen of the display portion 812 can beautomatically changed by determining the orientation of the portableinformation terminal (whether the portable information terminal isplaced horizontally or vertically). Furthermore, the direction ofdisplay on the screen can be changed by touch on the display portion812, operation with the operation button 813, sound input using themicrophone 816, or the like.

In a television device 7100 illustrated in FIG. 18A, a display portion7102 is incorporated in a housing 7101. The display portion 7102 iscapable of displaying images. The display device of one embodiment ofthe present invention can be used for the display portion 7102.Accordingly, a television device having a display portion with a highaperture ratio can be manufactured. In addition, here, the housing 7101is supported by a stand 7103.

The television device 7100 can be operated with an operation switchprovided in the housing 7101 or a separate remote controller 7111. Withoperation keys of the remote controller 7111, channels and volume can becontrolled and images displayed on the display portion 7102 can becontrolled. The remote controller 7111 may be provided with a displayportion for displaying data output from the remote controller 7111.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the use of the receiver, general televisionbroadcasts can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

A computer 7200 illustrated in FIG. 18B includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnecting port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured by using the display device of oneembodiment of the present invention for the display portion 7203. Thus,the display portion of the computer can have a high aperture ratio.

The camera 7300 illustrated in FIG. 18C includes a housing 7301, adisplay portion 7302, an operation button 7303, a shutter button 7304,and the like. Furthermore, an attachable lens 7306 is attached to thecamera 7300.

The display device of one embodiment of the present invention can beused for the display portion 7302. Thus, the display portion of thecamera can have a high aperture ratio.

Although the lens 7306 of the camera 7300 here is detachable from thehousing 7301 for replacement, the lens 7306 may be included in thehousing 7301.

Still images or moving images can be taken with the camera 7300 bypushing the shutter button 7304. In addition, images can be taken by atouch on the display portion 7302 that serves as a touch panel.

Note that a stroboscope, a viewfinder, or the like can be additionallyattached to the camera 7300. Alternatively, they may be incorporated inthe housing 7301.

This embodiment can be combined with any of other embodiments asappropriate.

EXAMPLE 1

Described in this example are a cross-sectional structure of a displayportion and a scan line driver circuit portion of a display device ofone embodiment of the present invention and the measurement results ofthe light transmittance of a contact portion where a transistor and apixel electrode are in contact with each other in a display region.

FIG. 19 shows the cross-sectional structure of the display portion andthe scan line driver circuit portion of the display device of thisexample.

The display device shown in FIG. 19 is an example of a transmissiveliquid crystal display device that includes a liquid crystal elementwith a vertical electric field mode.

As shown in FIG. 19, the display device includes the substrate 51, thetransistor 201, the transistor 206, the liquid crystal element 40, thecapacitor 219, the alignment film 133 a, the alignment film 133 b, theconnection portion 204, the adhesive layer 141, the coloring layer 131,the light-blocking layer 132, the overcoat 121, the substrate 61, thepolarizer 130, and the like.

The display portion 62 includes the transistor 206, the liquid crystalelement 40, and the capacitor 219.

The transistor 206 includes the gate 221, the insulating layer 213, theconductive layer 222 a, the conductive layer 222 c, and thesemiconductor layer 231.

Each of the conductive layers 222 a and 222 c is connected to thesemiconductor layer 231.

The liquid crystal element 40 is a liquid crystal element with a VAmode. The liquid crystal element 40 includes the pixel electrode 111,the common electrode 112, and the liquid crystal layer 113. The liquidcrystal layer 113 is positioned between the pixel electrode 111 and thecommon electrode 112.

The pixel electrode 111 is electrically connected to the semiconductorlayer 231 of the transistor 206 with the conductive layer 222 cpositioned therebetween.

The conductive layer 222 c functions as one of a pair of electrodes ofthe capacitor 219. A conductive layer 217 a functions as the other ofthe pair of electrodes of the capacitor 219. The conductive layer 222 cand the conductive layer 217 a overlap with each other with theinsulating layer 213 positioned therebetween. A conductive layer 217 band the conductive layer 218 are connected to each other.

The semiconductor layer 231, the conductive layer 222 c, the conductivelayer 217 a, the conductive layer 217 b, and the conductive layer 218are formed using a conductive material that transmits visible light. Theconductive layer 217 a and the conductive layer 217 b can be formedusing the same manufacturing step and the same material. The conductivelayer 218 and the conductive layer 222 c can be formed using the samemanufacturing step and the same material. Thus, the contact portionwhere the pixel electrode 111 and the transistor 206 are in contact witheach other, the contact portion where the conductive layer 217 b and theconductive layer 218 are in contact with each other, and the capacitor219 can be provided in the display region 68. Accordingly, the apertureratio can be increased.

Examples of materials and formation methods of the layers included inthe transistor 206 shown in FIG. 19 are described.

First, a conductive film that transmits visible light (e.g., ITSO) isformed as the conductive layer 217 a and the conductive layer 217 b, andthen, a metal film such as a Cu film is formed by a sputtering method asthe gate 221. The metal film functions as a scan line. With the use ofthis metal film, the gate 221 and a gate wiring of a transistor in theperipheral circuit can be formed in the same manufacturing step.

Then, a stack of a silicon nitride film and a silicon oxynitride film isformed as the insulating layer 213 functioning as a gate insulatinglayer. Then, a stack of a CAC-OS film and a CAAC-OS film is formed by asputtering method as the semiconductor layer 231. In the case where theCAAC-OS film with high chemical solution resistance and high plasmaresistance is formed over the CAC-OS film, the semiconductor layer 231is less damaged in the manufacturing process of the transistor. Then, anindium zinc oxide film is formed by a sputtering method as theconductive layer 222 c functioning as a source electrode or a drainelectrode. The semiconductor layer 231 and the conductive layer 222 ccan be formed by wet etching. In order to increase the selectivity sothat the semiconductor layer 231 is not etched at the time of formingthe conductive layer 222 c, the conductive layer 222 c is preferablyformed using an etchant different from that used for forming thesemiconductor layer 231. With the use of this indium zinc oxide film,the conductive layer 222 c and the conductive layer 218 can be formedusing the same manufacturing step.

Then, a metal film such as a Cu film is formed by a sputtering method asthe signal line and the conductive layer 222 a. With the use of thismetal film, the signal line, the conductive layer 222 a, and a sourcewiring and a drain wiring of a transistor in the peripheral circuit canbe formed using the same manufacturing step.

Then, the insulating layer 212 and the insulating layer 214 functioningas a passivation film are formed by stacking a silicon oxynitride filmand a silicon nitride film using a PECVD apparatus. Then, an acrylicresin is applied as the insulating layer 215 having a planarizationfunction, and an opening portion (a contact opening) is formed. Then, anITO film is formed as the pixel electrode 111.

Note that a Cu film that is formed as the scan line is preferably usedfor the gate electrode of the transistor included in the pixel. The Cufilm can suppress irradiation of a channel formation region with lightemitted from the backlight. In FIG. 7, the contact portion where thetransistor 206 and the pixel electrode 111 are in contact with eachother and the capacitor 219 can transmit visible light.

A layered structure capable of being used for a region 139 shown in FIG.19 was formed, and the light transmittance thereof was measured. FIG. 20shows the measurement results. Note that FIG. 20 also shows the lighttransmittance of glass (the substrate 51). The light transmittance wasmeasured with U-4100 Spectrophotometer (manufactured by HitachiHigh-Tech Science Corporation).

It is confirmed from FIG. 20 that the layered structure for increasingthe aperture ratio can transmit visible light. Accordingly, it issuggested that the power consumption of a backlight can be decreasedwhen a visible light transmitting material is used in the contactportion where the transistor 206 and the pixel electrode 111 are incontact with each other, the capacitor 219, and the like.

EXAMPLE 2

Described in this example are a cross-sectional structure of a displayportion and a scan line driver circuit portion of a display device ofone embodiment of the present invention and the measurement results ofthe light transmittance of a transistor positioned in a display region.

A manufacturing method of a transistor included in the display portionof the display device of this example is described with reference toFIGS. 21A1, 21B1, and 21C1 and FIGS. 22A1, 22B1, 22C1, and 22D1. Amanufacturing method of a transistor included in the scan line drivercircuit portion of the display device of this example is described withreference to FIGS. 21A2, 21B2, and 21C2 and FIGS. 22A2, 22B2, 22C2, and22D2.

First, a conductive layer 217 s is formed over the substrate 51, and aconductive layer 224 s is formed over the conductive layer 217 s (FIGS.21A1 and 21A2). The conductive layer 217 s is formed using a conductivematerial that transmits visible light (e.g., ITSO). The conductive layer224 s is preferably formed using a conductive material with lowerresistance than the conductive layer 217 s, such as metal. For example,a metal film such as a Cu film is formed by a sputtering method as theconductive layer 224 s.

Then, the conductive layer 217 s and the conductive layer 224 s areprocessed to form a gate (FIGS. 21B1 and 21B2). In the display portion,the island-like conductive layer 217 is formed (FIG. 21B1). In the scanline driver circuit portion, a layered structure of the island-likeconductive layer 217 and an island-like conductive layer 224 is formed(FIG. 21B2). The gate is preferably formed using a multi-tone mask (ahalftone mask, a gray-tone mask, or the like). In the case where amulti-tone mask is used, a gate that transmits visible light can beformed in the display portion, and a low-resistance gate and alow-resistance gate wiring can be formed in the scan line driver circuitportion without increasing the number of masks.

Then, the insulating layer 213 functioning as a gate insulating layer isformed, and the semiconductor layer 231 is formed over the insulatinglayer 213 (FIGS. 21C1 and 21C2). In this example, the insulating layer213 is formed by stacking a silicon nitride film and a siliconoxynitride film. In this example, the semiconductor layer 231 is formedby stacking a CAC-OS film and a CAAC-OS film by a sputtering method.When the CAAC-OS film with high chemical solution resistance and highplasma resistance is formed over the CAC-OS film, the semiconductorlayer 231 is less damaged in the manufacturing process of thetransistor. When an oxide semiconductor is used, the semiconductor layer231 that transmits visible light can be formed.

Then, a conductive layer 222 s is formed, and a conductive layer 222 tis formed over the conductive layer 222 s (FIGS. 22A1 and 22A2). Theconductive layer 222 s is formed using a conductive material thattransmits visible light. In this example, an indium zinc oxide film isformed as the conductive layer 222 s. The conductive layer 222 t ispreferably formed using a conductive material with lower resistance thanthe conductive layer 222 s, such as metal.

Then, the conductive layer 222 s and the conductive layer 222 t areprocessed to form a source and a drain (FIGS. 22B1 and 22B2). In thedisplay portion and the scan line driver circuit portion, theisland-like conductive layer 222 b and the island-like conductive layer222 c that are connected to a part of the semiconductor layer 231 areformed (FIGS. 22B1 and 22B2). In the display portion, the conductivelayer 222 t remains only in a portion corresponding to the island-likeconductive layer 222 a connected to a part of the island-like conductivelayer 222 b, so that a large part of the transistor transmits visiblelight (FIG. 22B1). In the scan line driver circuit portion, theisland-like conductive layer 222 a and an island-like conductive layer222 d formed by processing the conductive layer 222 t are provided overthe island-like conductive layer 222 b and the island-like conductivelayer 222 c (FIG. 22B2). The source and the drain as well as the gatesare preferably formed with a multi-tone mask. In the case where amulti-tone mask is used, a source and a drain that transmit visiblelight can be formed in the display portion, and a low-resistance source,a low-resistance drain, a low-resistance source wiring, and alow-resistance drain wiring can be formed in the driver circuit portionwithout increasing the number of masks. The semiconductor layer 231, thesource, and the drain can be formed by wet etching. In order to increasethe selectively so that the semiconductor layer 231 is not etched at thetime of forming the source and the drain, the source and the drain arepreferably formed using an etchant different from that used for formingthe semiconductor layer 231.

Then, the insulating layer 212 functioning as a gate insulating layer isformed, and the gate 223 is formed over the insulating layer 212. Thegate 223 is formed using a conductive material that transmits visiblelight. In this example, the insulating layer 212 is formed by stacking asilicon oxynitride film and a silicon nitride film with a PECVDapparatus. As shown in FIGS. 22C1 and 22C2, the gate 223 may be providedonly in the scan line driver circuit portion. As shown in FIGS. 22D1 and22D2, the gate 223 may be provided in each of the display portion andthe scan line driver circuit portion.

Thus, the transistor of the display device of this example can bemanufactured.

A layered structure capable of being used for a region 140 shown in FIG.22C1 was formed, and the light transmittance thereof was measured. FIG.23 shows the measurement results. Note that FIG. 23 also shows the lighttransmittance of glass (the substrate 51). The light transmittance wasmeasured with U-4100 Spectrophotometer (manufactured by HitachiHigh-Tech Science Corporation).

It is confirmed from FIG. 23 that the layered structure formed toincrease the aperture ratio can transmit visible light. Thus, it issuggested that the use of a visible light transmitting material in alarge part of the transistor in the display portion can reduce the powerconsumption of a backlight.

This application is based on Japanese Patent Application Serial No.2016-233560 filed with Japan Patent Office on Nov. 30, 2016 and JapanesePatent Application Serial No. 2017-099002 filed with Japan Patent Officeon May 18, 2017, the entire contents of which are hereby incorporated byreference.

What is claimed is:
 1. A display device comprising: a liquid crystalelement; a transistor; a scan line; and a signal line, wherein theliquid crystal element includes a pixel electrode, a liquid crystallayer, and a common electrode, wherein the scan line and the signal lineare each electrically connected to the transistor, wherein the scan lineand the signal line each include a metal layer, wherein the transistoris electrically connected to the pixel electrode, wherein asemiconductor layer of the transistor includes a stack of a first metaloxide layer and a second metal oxide layer, wherein the first metaloxide layer includes a region with lower crystallinity than the secondmetal oxide layer, wherein the transistor includes a first regionconnected to the pixel electrode, wherein the pixel electrode, thecommon electrode, and the first region are each configured to transmitvisible light, and wherein the visible light passes through the firstregion and the liquid crystal element and exits from the display device.2. The display device according to claim 1, wherein the first metaloxide layer and the second metal oxide layer each independently includeindium, metal M, and zinc, and wherein M represents aluminum, gallium,yttrium, or tin.
 3. The display device according to claim 2, wherein anatomic ratio of the indium to the metal M and the zinc is 4:x:y, where xis greater than or equal to 1.5 and less than or equal to 2.5 and y isgreater than or equal to 2 and less than or equal to
 4. 4. The displaydevice according to claim 2, wherein an atomic ratio of the indium tothe metal M and the zinc is 5:x:y, where x is greater than or equal to0.5 and less than or equal to 1.5 and y is greater than or equal to 5and less than or equal to
 7. 5. The display device according to claim 1,wherein the second metal oxide layer includes a crystal part, andwherein the crystal part has c-axis alignment.
 6. The display deviceaccording to claim 1, further comprising a touch sensor, wherein thetouch sensor is closer to a display surface than the liquid crystalelement and the transistor are.
 7. The display device according to claim1, wherein the scan line includes a portion overlapping with thesemiconductor layer.
 8. The display device according to claim 1, whereinthe visible light passes through the first region and the liquid crystalelement in the order presented and exits from the display device.
 9. Thedisplay device according to claim 1, wherein the visible light passesthrough the liquid crystal element and the first region in the orderpresented and exits from the display device.
 10. The display deviceaccording to claim 1, wherein a direction in which the scan line extendsintersects with a direction in which the signal line extends, andwherein a direction in which a plurality of pixels exhibiting the samecolor are aligned intersects with a direction in which the signal lineextends.
 11. A display module comprising: the display device accordingto claim 1; and a circuit board.
 12. An electronic device comprising:the display module according to claim 11; and at least one of anantenna, a battery, a housing, a camera, a speaker, a microphone, and anoperation button.
 13. A display device comprising: a liquid crystalelement; a transistor; a scan line; and a signal line, wherein theliquid crystal element includes a pixel electrode, a liquid crystallayer, and a common electrode, wherein the scan line and the signal lineare each electrically connected to the transistor, wherein the scan lineand the signal line each include a metal layer, wherein the transistoris electrically connected to the pixel electrode, wherein the transistorincludes a gate electrode, an insulating layer over the gate electrode,a semiconductor layer over the insulating layer, and a pair ofelectrodes over the semiconductor layer, wherein the semiconductor layerincludes a first metal oxide layer and a second metal oxide layer overthe first metal oxide layer, wherein the first metal oxide layerincludes a region with lower crystallinity than the second metal oxidelayer, wherein the transistor includes a first region connected to thepixel electrode, wherein the pixel electrode, the common electrode, andthe first region are each configured to transmit visible light, andwherein the visible light passes through the first region and the liquidcrystal element and exits from the display device.
 14. The displaydevice according to claim 13, wherein the first metal oxide layer andthe second metal oxide layer each independently include indium, metal M,and zinc, and wherein M represents aluminum, gallium, yttrium, or tin.15. The display device according to claim 14, wherein an atomic ratio ofthe indium to the metal M and the zinc is 4:x:y, where x is greater thanor equal to 1.5 and less than or equal to 2.5 and y is greater than orequal to 2 and less than or equal to
 4. 16. The display device accordingto claim 14, wherein an atomic ratio of the indium to the metal M andthe zinc is 5:x:y, where x is greater than or equal to 0.5 and less thanor equal to 1.5 and y is greater than or equal to 5 and less than orequal to
 7. 17. The display device according to claim 13, wherein thesecond metal oxide layer includes a crystal part, and wherein thecrystal part has c-axis alignment.
 18. The display device according toclaim 13, further comprising a touch sensor, wherein the touch sensor iscloser to a display surface than the liquid crystal element and thetransistor are.
 19. The display device according to claim 13, whereinthe scan line includes a portion overlapping with the semiconductorlayer.
 20. The display device according to claim 13, wherein the visiblelight passes through the first region and the liquid crystal element inthe order presented and exits from the display device.
 21. The displaydevice according to claim 13, wherein the visible light passes throughthe liquid crystal element and the first region in the order presentedand exits from the display device.
 22. The display device according toclaim 13, wherein a direction in which the scan line extends intersectswith a direction in which the signal line extends, and wherein adirection in which a plurality of pixels exhibiting the same color arealigned intersects with a direction in which the signal line extends.23. A display module comprising: the display device according to claim13; and a circuit board.
 24. An electronic device comprising: thedisplay module according to claim 23; and at least one of an antenna, abattery, a housing, a camera, a speaker, a microphone, and an operationbutton.